The picture's clear now: I need to unscrew the knob from the threaded bush, to expose the section with the grub screws - and then re-tighten those.
Unfortunately, maximum application of brute force has achieved nothing, other than to shift the whole shooting match around the stick by a further 70 degrees or so, such that the "H"-gate is now at right-angles to straight ahead. The knob is still loose as well.
The knob appears to be seized on the threaded bush - or else someone has Locktited it on. All I'm succeeding in doing (I suspect) is chewing up the top of the gearstick. I think I'm going to have to bite the bullet and let Berrybrook have a go at it. It's looking like judicious application of a hacksaw, and a new gearknob for me
