Andy Pandy wrote:

It's pointless reporting it to the police as for a start the OP can't
be sure it *is* fraud (it could simply be a mistake),

OK, that's possible. But if so, it should be straightforward to sort out.

and secondly if
it is fraud, it's the retailer or the bank that's the victim so it's
up to them to report it.

This is not necessarily true. If the bank are being so obtuse and
unhelpful as to make the OP suggest they are actually "assisting" the
fraudster, then if the cardholder cannot get the charges reversed,
*he* is the victim. Moreover, it's not just the fraudster who
would benefit, but the bank and card company too, from the transaction
fees involved. Thus if there is fraud, and the bank and card company
are facilitating it by being less than enthusiastically helpful to
the cardholder, they would automatically become accomplices.