Thanks to those who helped. A new timer arrived today and confident that I understood the circuit I quickly fitted it.
Shock - horror - the compressor wouldn't operate.
I refitted the old timer - manually wound it round - still it wouldn't operate.
Out with the meter - found the circuit to be open circuit to the motor. What had I done????
Took the terminal cover off the compressor to see if I could track down the fault - plugged it in again - and it fired up.
Had I disturbed a connection?
No - I did the job so quick and the motor was so hot that the compressor stalled when I applied power and then tripped the overheat trip.
Working fine now.