I nearly had it going, then something failed and I am back to a completely blank screen. So I have bought a logic tracer from woodchips (on this forum) which I'm going to use to try and diagnose the problem. On the way I have learned an awful lot about the hardware and ROM, and it is all documented in that thread.
If anyone reading that has any ideas, well, you know... I would be very happy to read them!
The main point is that I will get it working. I've been beaten by broken hardware before, but not this time.