Thanks Mick and Steve.
It was indeed the capacitor. One of the terminals had corroded and dropped off. I think it was shorting to the other capacitor terminal so there was no phase difference on the start windings. I think that stopped it reaching enough speed to open the centrifugal switch when I started it by flicking the pulley.
Fortunately there was a spare terminal on the capacitor

. Unfortunately it too was corroded and broke when I pushed the spade on.

.
I soldered a new terminal on and reassembled. Unfortunately the capacitor leads are so short that I had to take the motor off for the repair but it's all working again now.
I hope to finish the back plate tomorrow.
Russell