Thank you so much, Jean.
I think it's fairly likely that my Mary Ann was admitted to the workhouse and asylum. They were situated in her locality, and she is described, as on 1851 census, 'housewife', of which there weren't many in labourers' households.
1) I've gone through as many 'sound-alikes', transcription errors as I can think of, fairly exhaustively. The GRO can't find any close matches, but they have reminded me that registration wasn't compulsory until 1874.
2) I looked at the possibility of emigration, even bought world membership of FMP to do so. Nothing helpful. I also looked at the possibility of him serving at sea (see below).
3) I sent for possible Edward Cain death certs from that time. Results were wrong age or very wrong occupation.
4) Possible but unlikely. I think Ed's mother Mary Ann would have intervened. There is a Mary Anne in the household born same year, but that's his sister!
There are criminal records for an Edward Cain (without age of course!), but I can't find him amongst deportees.
* Edward was name of Mary Ann Boyne's father. Thomas (bro) was name John Cain's father. William was family name incl JC's grandfather.
I do need to follow Mary Ann's siblings and look at Robert's death. Can't find my note on this but I think bro Thomas died in 1850s.
2 possible records fot Wm in 1871. 1st is carriage.maker in Ardwick, Manc., but why would he go back to labouring in brass foundry? (I think this one's father was a James Cain)
Other is 'Boy 2nd class age 16(?) born M/C aboard HM training ship Ganges(1) in Plymouth Sound. I believe there was at the time a govt scheme for children of the poor to be trained at armed forces establishments, May explain how Wm was literate enough to sign marriage cert in good hand.
Thank you for your help. It's made me go through assumptions and reasons, and made me question again, when I was about to lose heart!
Dave