Mother keeps having money stolen

That's distressing for you, sorry to hear this

Ref the card details; when I spoke to my mum's bank about her suggestion that we needed a joint account in case she went into hospital and she couldn't get to any cash, the bank said "she could just ask you to use her card and PIN, if she trusts you" - I asked if this would be within the rules and they said yes

I would suggest asking the bank for some consecutively numbered notes and then reporting the numbers to the police if they go missing - if it is a simple misunderstanding then no harm done

The amount involved sounds similar to the amount my elderlies have wanted to keep by them to keep feeling independent, I wouldn't suggest trying to restrict that, just try to catch or deter the tealeaf - perhaps even by putting a note with the cash saying "the serial numbers of these notes have been recorded for reporting to the police in the event of theft"

Or setting up a notebook for all legitimate callers to record any money she gives them, so they know that it is being looked into "Dear visitors/carers, please note in here all cash paid to you by mum, as she has lost track of some recently so it is necessary to account for her cash formally now"

HTH