It seems to me the police, even in 1982, would have considered foul play and looked into the Septic tank.
I don’t know how British septic tanks are built, but there should have been a ‘clue’ of freshly disturbed earth...............
Some septic systems have a manhole-like opening that a person might fall into, so you wouldn’t need to dig it up to get someone in there.
Heck, it’s even conceivable she fell in one night after a few too many, stumbling in the dark basement, and he really is telling the truth that he just woke up one morning and she was gone.