Chris
There used to be a group of archeology guys who called themselves Severnside aviation society, who I think were based in Portishead.
I wonder if they are still around? they would be a good source of information.
Remember watching them recover the remains of a Wellington bomber in a field between Bristol / Bath - they ended up with a 20' hole and alot of bomber. That was in the mid 1980's though.
Having flown over the Bristol Channel a number of times I have never seen anything, though TBH I did not look too hard down below.
I'll have a better look next time i'm over there.