Again, this blog is exploring the mystery of Paul's disappearance and replacement, but, if you believe that John was also replaced, there is a document that you would want to be able to see in the UK National Archives: IR 59/1291. It contains Death Duty Accounts for John Winston Ono Lennon (John Lennon) and unless Baby Boomers are planning on living for a really, really long time, we won't be able to see the contents because the document's opening date is January 1, 2076.
The importance of seeing details of the document is to know WHICH John Lennon died in December, 1980. The Death Duty Accounts name hiers to the decedent and their respective inheritance.
The replacement John Lennon changed his name--apparently by a British device called "deed poll"-- on April 22, 1969. He changed his name to John Winston Ono Lennon. Unless the man wants the information in the deed poll known, it would never be published.
An interesting "reason" The Archives makes for not releasing the document is that the youngest beneficiary would be 22 in 1997. That would most likely be Sean and that would mean that the replacement died because Sean is the child of Yoko and the replacement.
The Archives lets you browse to see other well-known Britons and lets you know that their files are closed for a long time, too. For instance, actor and comedian Peter Seller's file is closed for 84 years. But I browsed through the list and couldn't find a British illuminary of the stature of John Lennon. And I don't think that's my age bias.
SO . . . all you John researchers should consider noising about the need to see some details of IR 59/1291. Maybe they could use a black magic marker on the confidential parts like they do in US Foia requests. It seems to me if we knew who the heirs are, we would know who the John was.