Thank you, Ben and Kris, for your interest. To answer Ben's question first - No, in the very limited online resources available for Devon, i have failed to find either a death or a marriage for the other Benjamin until now. However, you will see that Kris has now been able to fill that gap for me.
For which, Kris, I am very grateful. I'm afraid that the possibility of emigration had not occurred to me. It's hardly surprising that I had failed to find them in the UK 1851 census, is it?
A marriage witness is of course what I was hoping to find, though I might have wished for a less common name than John! Abel and Elizabeth had children christened at Broadwood Widger every two years from 1775 to 1785, but there was no John among these.
The Benjamin who was born to John and Frances did on however have an older brother John, christened in 1773. This would mean that he was in his late 40s when Benjamin and Charity married. On the other hand, it looks possible that he may have been the only surviving brother. He had I think married Grace Cory at St Stephen by Launceston in 1798, and they were still producing a family at Broadwood in 1816.
So far as this is a pointer, it suggests that the Benjamin who went to Canada might not have been the son of Abel. I think perhaps that my next move might be to try to contact the authors of the Canadian website, and ask if they have any evidence for their conclusion.
Meanwhile, thank you again for the trouble you have taken.
Regards, David.