OK, here goes: the reason for the failure to get any further receive interrupts was that the RXD1 pin was being re-configured as an output!

I was using some diagnostic routines to output a value to some LEDs, some of which were on Port 1 and some on Port 3. These routines were not using the read-modify-write instructions that read the port latches instead of the pin.

I had inherited the code and not looked too closely at these routines on the basis that they seemed to be doing what they were supposed to (after all setting a port pin isn't complicated). Also these are used for debug only and was using the philosophy 'if it ain't broke, don't fix it'. Unfortunately, it was broke.

Thanks for all suggestions.

Kevan