OK, its all done....it was the solenoid on the DC motor.
Turned it on checked there is power to the solenoid and power to the solenoid coil, but no go.
Disconnected the power cables from the solenoid and connected together - it runs.
Took off the solenoid, all the connectors were corroded. Measured across the coil connections -Megohms- took it apart and inside one of the coil wires had broken off the connector.
Cleaned everything, re-soldered the coil wires, replaced all the connectors on the wires leading to the solenoid. Put it all back on and it runs !!
Note the solenoid is held on the motor with two bolts and nuts through the end cap, easy to get off, but impossible to put back on without removing the end cap, which is also not possible without removing the motor. I reassembled it to the motor with large self tapping screws and Loctite thread locker.
I will source another 24VDC solenoid to replace this rusty thing and mount it somewhere else.
Photos uploaded of the watermaker and the disassembled solenoid for your interest.