This is yet another reason why I’ve been trying to get Monzo to surface the “finalised available balance (excluding pots)” that they calculate overdrafts with. Unfortunately, Monzo (rightly or wrongly) pretend pending transactions are finalised with Mastercard and show an incorrect balance accordingly. I’ve wrote a lot about it here: Pre-ordering authorising the full amount months in advance
For what it’s worth, the problem @barneylodge is having would be much less of an issue with most other bank accounts where pre-authorised amounts are not taken off your balance in the same way, and certainly don’t appear the same as a genuine overdraft.
In the email conversation I’ve had with Monzo, they’re aware that this figure needs to be made visible in the app somehow both for those who are genuinely using overdrafts and need to know when they’re going to be charged, and also in situations like this where a pre-authorisation is made but never finalised (or finalised with a different amount).