Well then, Operation Melvin The Stripy Soggy Submarine is progressing. After many failed attempts and an awful lot of language too strong for here I got the seal to go down properly(ish, it's got a tear in it), I've packed all the bellows and round the rear seal with silicon grease, and it's been for a bath run.
It doesn't leak anymore, and feeling adventurous I tried a couple of dives. All works well, even if it does come up bottom first...
A story is nothing without pictures, so here goes