I would imagine the force and severity of approach they would take to prosecuting you for £40m would be a lot lot stronger than £400.
The level of information they have access to is limited. They can't see when you've checked your bank balance, they can only see the transactions, i.e. they can't verify whether you were aware of that unless you spent specifically £400 more that month than you otherwise would, using that account. They certainly couldn't link a random forum post to IP addresses or anything (they probably could for larger cases but not the sort of thing they'll do for a £400 bill). As it stands this thread would not be permissible in court (IANAL!).
If you did spend it immediately on a fine 1962 chateau-lafite and some lagavulin, using your debit card, and then stick your fingers up to them, you won't get a criminal record either, you'll be asked for the money back and you could probably get away with paying less back.
I do actually have some experience with a very similar case when natwest transferred £2500 into my account mistakenly when I was a teenager. I spent a few hundred of it, they asked for it all back, I told them I didn't have it, they just accepted what I did have left.
Leave it in there, plead ignorance.