Well, this one's sorted. Not only that, but what was previously ALWAYS a sluggish starting engine now fires on pretty much one crank!
Thanks in no small part to the advice here, I searched for a starter specialist - well landed on my feet to find these guys on my doorstep: http://www.startermotorsalternators.com/
Rebuilt, rewound, new solenoid, new internals, even resprayed, for a great price, and the owner of the firm couldn't have been more helpful - he even let me collect the starter from his house as he was shut on Saturdays. Oh, and he did it in 24 hours. The previous rebuild was carried out by a marine specialist, took two weeks, and lasted six months!
So, I can't recommend these chaps enough. They have an eBay shop so can probably help anyone, anywhere.
Thanks again for all the advice here. An interesting week!
Chris