This was diagnosed as either a low battery, loose battery contacts, a fault with the solenoid or the motor itself. The first cause was eliminated by checking the battery state of charge, which was fine. The second cause was also eliminated by checking that all the battery connections were tight and in good order - they were. I then disconnected the cables from the solenoid (photo 1) and removed the solenoid from the starter motor. There was a slight looseness in one of the the electrical connection studs on the top of the solenoid (photo 2) which was corrected by carefully tightening the lower of the two nuts on the stud (not shown in photo 2 but just visible in photo 1). Unfortunately, it's not possible to take the solenoid apart, as it appears to be a factory sealed unit, so it wasn't possible to check the condition of the internal contacts.
|Solenoid in place on the starter motor|
|Solenoid removed, showing electrical connection studs|