It is clear the bank has had a severe exfiltration event. There are other reports that online. IMHO the law should make banks report breaches to the ICO and a record of the nature and size of the breach be public.
Through the process I learned that in the UK you can call 159 to directly contact your bank fraud dept (most banks) https://stopscamsuk.org.uk/159
I also learnt about the police’s Action Fraud hotline to report cybercrime. https://www.actionfraud.police.uk/what-is-action-fraud
The phisher was very determined. They called back in 15 minutes claiming to be from the bank fraud dept returning my call. Then 2 weeks later they called back claiming to be from Action Fraud.
However prepared you think you are for such an attack, my advice is to have utmost caution for every single call from anyone claiming to be anyone.
I also have a Monzo account. Even if they called me I wouldn’t use this. Hang up. Call them. Don’t let them call you.