Ok..so a little update. After the advice to remove ALL the Cybergraphics stuff I had a bit more luck. Following that I reinstalled Picasso96 and rebooted.
Huh! It didn't crash. I tentatively moved my mouse - still ok. Dared to open up Screenmode - lots of nice new modes to play with! Chose one (16 bit 800x600), Saved, the TV went blank grey and the monitor showed a crisp, large Workbench. I allowed myself to let out a whoop of joy and then the computer crashed. Software Failure 80000004.
I was still reasonably pleased though because it seemed like a bit step forward. I started again, this time I made sure that I loaded Picasso96Mode and attached the settings file to the graphics card (went to the menu Attach to Card and told it to disable all the drivers outside of the hardware capability of the Cybervision. At least, I think that was what I was doing). Then, after saving that and resetting I tried again. This time the PC monitor displayed half an Amiga workbench before it obviously again crashed.
It is also flakey in standard PAL modes. I was able to run the Debug program and it gave a full report of the card so it seems to be operating ok which is a relief.
Now, is it the software or my Amiga 4000 which is the problem? Hopefully it is still a software issue but I do have Buster 09 (surface mounted!!!).

Someone mentioned Cybergfx 4. I'm more than happy to buy it if I can be sure it'll work. Are there any time delay demos you can try?