Right, fixed it this morning.
I replaced and upgraded both locking fuses, 57 and 58 from 15A to 20A. Then I disconnected the battery to reset the locking module and all is now sweetness and light! It works as it did on Sunday evening.
(My wife has since admitted to unlocking/locking using the dash button several times in a row having got confused as to whether it was or was not locked! This obviously blew fuse 57.)
After posting on here yesterday I did a little more googling and found a US site that suggested disconnecting the battery to reset the system after replacing the fuses and it worked.
The upgrade of the fuses from 15A to 20A is a BMW recommended service 'fix' after many early E90/91's central locking failures. Why BMW didn't change these fuses during a normal service to prevent the problem happening on later cars and save their reputation is beyond me! Still that's big headed BMW for you.
Anyway all working now.