Good news... using the logic probe I can see the now narrowed pulse at pin 12 of IC29 but alas it was missing from pin 9. I changed C29 for one I had (which is remarkable in itself) and the system now starts as expected. I see the message as before except now it asks for a system disk in drive A.
I will now reassemble the system with the Winchester and see how far I can get. Naturally I will keep you posted. Thank you so much for all of the help, the details you have given me here have once again saved the day.
John: As soon as I can get to a Basic, I will run the program you posted.