Thanks for your reply Maggi. All that I have so far are the years, which are 1973 for Edith and 1986 for John.
I have established that Edith was my great-uncle's daughter (I found the birth in the St Catherines Index in 1905).
Is there any chance, maybe, that you know which papers might have published an obituary? There is a Welsh edition of the Liverpool Daily Post, but I think that obits would be more likely to be in a more local paper. Would you agree?
I've, so far, been unable to find a marriage for Edith and John, but it must be recorded SOMEWHERE. Blowed if I know where, though!