I wanted to make a constructive contribution to this topic to help others and answer the question. ( @markgengine’s original post)
Unfortunately, the topic was closed before I was able to contribute. I’ve set out some information below that hopefully helps others in a similar situation be it with a Monzo card or one issued by another organisation.
Police No Longer Record Lost Property
Since Monday 1st October 2018, Police Forces across the United Kingdom stopped taking reports, at front counters or via 101, of lost property from members of the public.
This national move comes after discussions between police forces and the National Police Front Counters Forum (NPFCF).
There is no statutory duty for police forces to maintain a system of recording non-evidential property (lost and found) and, in a bid to reduce non-essential demand and associated costs and bureaucracy, a proposal to cease the recording of lost property reports was approved by the NPCC (National Police Chiefs Council).
Reporting Lost / Found Property
Lost property can be reported at any at a number of places online for example:
Report My Loss provides a printed numbered ownership certificate, confirming the details of a lost item to be given to insurance companies in the event of a claim. They do charge a fee for this service.
Report Lost Property
Lost Debit, Credit and Store Cards
Report the loss directly to the debit/credit provider. They will see to cancelling the cards and be able to issue a replacement card.
My advice would be, on finding a lost bank card, to destroy it. It’s safer for the bank to issue a new card to the cardholder.