Some progress (if you can call it that). after doing absolutely nothing except leaving it overnight the error has changed to 4 beeps indicating an error in sound rom 4. The rom is seated correctly and meters ok pin for pin with the other roms (bar the /E pin).
I'm wondering if during storage some damp has somehow caused problems. Has anyone come across this before? I'm going to leave it powered up for the rest of today to let it warm through and see if that helps.
If it turns out that rom4 has gone belly up, where can I obtain a rom image so I can burn a new one?
Mike.