It worked
The focus assembly has been saved and is now fully fitted back on the chassis.
The focusing is good and the picture centres nicely.
It took me a while to establish the north and south poles of the copper wire coil because with the 47 R resistor in series there is limited current in the coil but if you hold a compass very close to the coil you will see the pointer move.
I wound just over 20 turns around the magnet and it got quite warm after 3 or 4 flashes across a car battery.
Thank you very much argus for telling me about this.
Robin