All your comments are valuable, but can we get back to the fundamental problem - I can't get serial port 1 to interrupt on the RI1 bit at all:

1. I know the RI1 bit is being set, because I can test it and receive bytes using it without interrupts.
   
2. I know the baud rate generation, serial port mode etc. are set up correctly for the same reason - I can send and receive bytes correctly by testing the RI1 and TI1 flags.
   
3. But when I enable interrupts and wait for an interrupt, nothing happens. My interrupt handler is never called.
   
4. If I do the same thing using serial port 0, it works fine.

I deduce that there must be something that's different between the serial port 1 and serial port 0 interupts, and that I've got something set up wrong. But what?

Many thanks - Rowan