Yep, it turned out to be even more difficult than I had expected! In the end all I managed was to get the window up but I'm very happy with that as there is some windy wet weather in the post!
I was unable to access the regulator assembly before darkness beat me! It looks like you need to take the window pane out before you can remove the metal inner door panel to get access to the regulator - any idea if this is correct? However, to get the window pane out, the window pane needs to be aligned with the holes in that metal inner door panel, so that it is possible to loosen the bolts, which hold the window pane in place. However, my window was stuck about 75% open, so I couldn't align it, so I couldn't free the window pane, and so I couldn't remove the inner metal panel! Catch22.
In the end, as a last ditch attempt, I removed the motor and tried tapping the plastic regulator mechanism (as I had read another AO had tried with success) with the rubber handle of a wrench, in the hope it might joggle something inside enough to get the window moving! I re-attached the motor and switch and tried to power it up. At first it didn't budge much but then I lowered it a fraction, pulled the pane while operating the switch and it gradually ground and grated its way up!
I'll have a cold beer this eve and tackle again some day when the weather permits. I'm still not sure exactly which part of the assembly/mechanism is toast, but I imagine its going to require the whole new regulator assembly anyway. Thanks again for the pointers.