Just to report back, The ombudsman has upheld my complaint against Monzo. Monzo acknowledged their errors in their investigation and agree the charge shouldn’t have happened. If they agree with the decision, I’ll get my overdraft fee refunded, my credit file amended so it’s as if it was never used, and an additional £50 compensation.
They can’t however enforce Monzo to change the way they handle their clocks or amend the overdrafts agreement to make note of this, or explain it to the user as part of the flow, and not just in an FAQ somewhere. That’s a matter for the FCA if I want to pursue that.
Monzo have said if it happens again to notify them and they’ll put it right.