I here republish a revision of an article which has appeared before, I think thrice, on my blog. (Some corrections and additions offered in the original threads have, with thanks, been incorporated into this text.) People who just visit this blog ... and, indeed, the Internet generally ... for a quick giggle need only go straight down to the brief paragraphs in blue.
I am particularly grateful to people who enable me to correct any misstatements.
As you enter Westminster Cathedral, you will, if you look at the wall to your left, see two large sheets of brass (bronze?) which purport to give us a list of the chief pastors of the Catholic Church in this country from S Augustine onwards, showing their communion with the See of S Peter. (Who compiled it? See the thread. Interestingly, it claims that the Vicars Apostolic of the London District were chief pastors during the penal days ... is this true?) The aim of this list is surely ecclesiological (indeed, polemical and anti-Anglican) and designed to make a claim for the status of the Roman Catholic Particular Churches in England based upon their Communio with the See of S Peter. Such a public witness and explicatio of Communio must clearly be held to embody the formal teaching of the Particular Church of Westminster, God bless her.
What I am interested in is the early fifteenth century, the time when the Great Western Schism had not yet been resolved. There were at one point three simultaneous, competing, 'lines' of 'popes': the Roman Popes; the Avignon Popes; and, after the Council of Pisa in 1409 deposed both of them (their depositions were not then accepted by either of them) there were also the Pisan Popes. Of course, dogmatic purists will reassure us that really there can only be one pope. One of those three prelates was the real pope; the other two were antipopes. Obviously, people who adhered to one of the two antipopes, believing him to be the true pope, were in completely good faith and most earnestly desired to be in communion with the Successor of S Peter. An argument which attempted to portray them as 'non-papalist' would be dishonest. But objectively such adherents were as a matter of fact not in communion with him; they objectively were in schism from the one man whom God (alone!) knows to have been pope.
Dr E L Mascall observed that there had never been a definitive judgement on which of the three was the genuine 'line' of 'real' popes, and different editions of such works as the Annuario pontificio are not always in agreement; but the de facto consensus is that the Roman popes were the Real Macoy. Down to 1409, that is. 1409 is the year the real fun starts. Are you sitting comfortably?
As the year 1409 began, the Roman pope was Gregory XII. England was in communion with him. Scotland, France, and Spain, on the other hand, were in communion with the Avignon pope, Benedict XIII. But, during that year 1409, most of the cardinals of each 'pope' deserted their respective masters and, in the council of Pisa, came together; and claimed to depose them both and to elect a new pope, Alexander V. 2+1=3 popes! Now let's see what the official Westmonasteriensian lists do with this situation.
The lists in Westminster Cathedral show Gregory XII (Roman) as becoming pope in 1406; then Alexander V (Pisan) in 1409 (although the 'genuine' and 'Roman' pope Gregory XII did not abdicate until 1415).
In other words, the Church and Bishop of Westminster, interestingly, by implication proclaim the Pisan doctrine that a 'Council' unlawfully convoked by a group of cardinals in collaboration with some 'schismatics' (as Pisa was) and without the consent of the lawful Roman Pontiff, can lawfully depose the lawful pope (in this case, Gregory XII) and lawfully elect eo nondum defuncto a lawful replacement.
(This Westmonasteriensian-Pisan doctrine is distinctly thought-provoking! Would four Cardinals and three SSPX bishops, gathered in solemn Conclave at the top of Westminster Cathedral's preposterous Minaret, suffice for validity?)
However, Alexander V died in 1410; and his Pisan 'line' was continued by the election of John XXIII (sic). But the Westminster list does not mention this John XXIII. The next pope it gives is Martin V, who was to be elected by the Council of Constance in 1417. (At that Council, both Gregory XII [Roman] and John XXIII [Pisan] did either accept deposition or abdicate.)
So it appears that, from 1410 until 1417, according to the public teaching of the Church of Westminster, the See of S Peter was vacant. But it is unclear why, in this public teaching of the Church of Westminster, Alexander V (Pisan) was truly pope but his immediate lineal successor John XXIII (Pisan) lacked the same status. Obviously, the idiosyncratic dogmas of Westminster have profundities which I have not yet plumbed.
A seven-year interregnum, in which nobody is in communion with a pope because there isn't one, is surely long enough to raise interesting ecclesiological questions. I return to this in Footnote (3).
So far we have been considering the papal names on the left side of the Westminster Cathedral list. Let us now look to the right, where we find the Archbishops of Canterbury listed and the date (if known) when they received their Pallia. The anonymous begetter of this list rightly takes granting and reception of the Pallium to be a clear indicator of Communio between Rome and Canterbury. And in 1414, Henry Chichele became Archbishop of Canterbury and, that same year, received the Pallium at Kings Sutton. Yes ... he received the Pallium ... in ... get this ... 1414.
In 1414, most of the world, including England, regarded the Pisan pope John XXIII as the true pope. Only Italy still adhered to the Roman pope Gregory XII. (Remember: the Church of Westminster regards the See of Rome as being vacant from 1410 to 1417; incidentally, in case you were wondering, the Avignon pope, Benedict XIII, had fled to a small Spanish town called Peniscola and by this point was ignored by everyone everywhere else.)
So who sent the Pallium to Archbishop Chichele in 1414?
I have no doubt that it was the Pisan pope John XXIII. (See the thread.)
But the Church of Westminster officially and very publicly dismisses this poor chap as non-existent, i.e. by implication as a mere antipope. As do modern lists of the popes.
So, when Papa Roncalli was elected Bishop of Rome in 1958, he took the title 'John XXIII' as if there never before had been a lawful pope of that name and number.
Irrespective of the question whether John XXIII (version 1) was or was not truly pope, I find it hard to understand how the Church of Westminster thinks it is demonstrating the importance of links of communion between the chief pastors of the Catholic Church in this country and the Holy See by boasting that Archbishop Chichele received the Pallium at a time when its own list declares the See of Rome to have been vacant, without there being any lawful pope (in the Westmonasteriensian view) qualified to bless and send out Pallia.
Stigand, incidentally, raises similar questions for Rigid Westmonasterialensian Extremists.
FOOTNOTES: (1) All this would be even greater fun if a Catholic Cathedral in Scotland had a parallel list ... also writ very large in brass (bronze?) ... showing the quite different list of 'popes' with whom the Scottish dioceses were in communion between 1378 and 1409, and who, I imagine, sent Pallia to Scottish metropolitans. The Avignon Pope Benedict XIII conferred University status upon the Schola at S Andrews in 1413 ... he is still honoured there. I wonder which papal claimant the town of Berwick on Tweed held communio with! And how about the Medieval diocese of Sodor and Man, which in any case showed a tendency to episcopal duplicity in the Middle Ages? And there are our beloved Channel Islands, happy little sunlit tax-havens and historically parts of the Diocese of Coutances. There could be industrial scope for manufacturers of big brass plates to make money by producing contradictory successio lists!
(2) I would not like anybody to think that I am mocking the teaching of Holy Mother Church, defined as tenendum de fide at Vatican I, concerning the Petrine Ministry; or that the facts about the Great Schism of the West in any way whatsoever throw the least doubt upon that teaching, which I have spent my whole adult life asserting and defending. They most certainly do not. In my view, the theological problems which are indeed thrown up by the Great Western Schism are easily, and best, dealt with by applying principles laid out in Paragraph 17 of the document Communionis notio (1992 AAS 85) issued by the CDF under Joseph Cardinal Ratzinger. And, indeed, any narrative of the Great Schism indicates the need for just such a nuanced understanding of Ecclesiology, of Communio, and of schism, as Ratzinger gives there. (See also his thoroughly admirable Dominus Iesus  Paragraph 17. I cannot help feeling that this is an attitude which Professor Dr E L Mascall, who cogently raised the ecclesiological problems thrown up by the Great Schism, would have been able to adopt.)
(3) Another approach would be to argue that fundamentally it is with the Roman Church, not with its Bishop simpliciter, that Christians are technically obliged to be in Communion. This would also solve another problem raised by Fr Mascall, that of periods of papal sede vacante ... which, in the Westmonasteriensian view, can go on for at least seven years without any insuperable theological problem arising ... during which, of course, nobody is in communion with the pope because there isn't one, but Catholics are all in Communion with the Roman Church because that does not cease to exist. Readers will also remember that two of the earliest witnesses to the Roman Primacy, S Ignatius and S Irenaeus, refer to the Church of Rome without actually mentioning its Bishop; and that the earliest known exercise of a primatial ministry is the 'Epistle of Clement', which is written as from the Roman Church. Of course, Rome's primacy is necessarily going to be exercised by the Bishop of that Church, who justly is held to be S Peter's Successor. But, in the end, I propose, Rome is not the primatial Church because it has the Pope as its Bishop; the Pope is the primatial Bishop because he has Rome for his Church. I put this forward as a personal speculation which seems to me to resolve some of the problems.