Finally solved the mystery of my loose (but unremoveable!) Roadster gear knob, thanks to Jays, who sent me his spare one. Once I had a replacement to hand, I set about my old one with a pair of stilson wrenches and effectively destroyed it. It took some removing.

And the cause of my distress? The boss, bush, whatever-you-like-to-call-it of my old gearknob was fitted upside down on the stick - so that the three grub screws were concealed
inside the knob, and then the knob itself had been
Loctited on. Beat that for lack of foresight!
What I'm wondering is ... was it the factory, a dealer, or the first owner who fitted it thus - bearing in mind that the car had 250 miles on the clock when it came my way? Whoever it was - they should know better