Been doing a bit of testing, because I wanted to be absolutely sure that the battery wasn't the cause of the problem.
With the engine kill switch OFF, I tried operating the starter:
1) It turned slowly for a couple of seconds then stopped (probably on a compression stroke). Subsequent attempts resulted in only clicking solenoid.
2) Kicked it over slightly to get past TDC, then tried the starter again. Same as before. Worked slowly for couple of seconds, then just clicking solenoid.
3) Kicked it over slightly to get past TDC, then tried the starter again. Now, all it will do is click the solenoid.
So, it could be the battery!
But, the headlight still illuminates brightly.
4) connected jump leads to my car battery and tried starter again. Now the starter works again, faster than before and keeps going as long as I want (in short spells of about 5 seconds).
Hmmm. So it could it could be the battery then !!
5) Decided to try again without the jump leads. Jump leads had been connected for only about 2 minutes, but now they are disconnected. Starter continues to operate the same as it did when the jump leads were connected. Ie it works at medium speed and is consistent.
Hmmm. Surely having the jump leads connected for only 2 minutes did not recondition the XBR battery sufficiently to make this much difference?
6) Tried again with jump leads disconnected and suddenly, the starter speeds up to the proper speed. It continues to operate at the proper speed for lengthy periods (at least 5 second bursts) even after repeated attempts over the space of 10 - 15 minutes. No sign of slowing. It's working better than it was when the jump leads were connected!!
So, the battery is strong!
What could be wrong with the starter motor that makes it sometimes not work at all, sometimes works slowly and sometimes like it should?
Can't see any loose or corroded connections.
It did cross my mind that it just needed to be used; perhaps dirty contacts in the commmutator or something. But thinking back to just before the problems began sometime last summer / autumn, the starter had been getting used regularly and it had been working consistently well.
Hmm....what to do.....?