Thanks Tabetha,

That has clarified it greatly for me. and I can see now how the solenoid "throw" being weak would lead to reduced power to the starter motor. Sounds to me like that is the problem.
I will check out that connection before I resort to a new starter. I can get a brand new Lucas unit for £50 with a years ticket, so at least I have a fallback.
Thanks for a great response !