Fit the battery and pull the coil lead off then crank the motor over to see if it dies quickly or sustains a good voltage.

Thanks for all the replies but its not the starter motor after all.
Looks like I need a new battery,  charged it overnight and turns over fine this morning.

FWIW, this was the symptom I had; spins well for a few cranks after full charge, then it rapidly gets slower and only just manages to turn the engine.  Charge the battery, and all's well. Just like a failed battery!  It may well be worth popping the starter off and checking for radial play in the shaft, and the condition of the bush (a bush peened or heavily worn on one side will indicate too much starter shaft play).  Might save yourself the cost of buying a new battery unnecessarily.

About 40 secs of cranking with no drop off of speed and volts to a shade under 12, then climbed back to 12.4 quite quickly, so I guess battery is basically OK

I'll just hang the conditioning charger on it more often

Negligable side play on starter and bush looks ok

could have been a bad connection at the solenoid battery feed terminal  and removing and refitting them has improved the contact.

I reckon the battery was weak as the remote for my lights wouldn't work,  fine once battery charged.

I'll check the alternator connections in case it's not charging as well as it could.