Aha, sunroof is diagnosed and partly fixed now

The motor gearbox isn't stripped but the outer gear is a bit chewed and has tight spots, so will try and locate a similar enough replacement that works. Also the cables are fantastic, not a spot of rust and nicely lubed up. As I said, there's evidence this car has been very well cared for until recent years. Just glad we caught it in time before it got any more abuse & neglect
Main problem now though the plastic 'sliders' for want of a better word, that run in channels & are attached to the cables, either side of the roof have been damaged. I'll try and get a photo as it would be easier than trying to describe exactly which bit, but the pivots have been broken off said sliders and this has left the mechanism with a lot of play & allows the associated parts that attached to them to come loose sometimes.
Only trouble is, I'm not sure if these will be replaceable without dropping the cassette which I'm loathe to do now as I feel headliners are never quite the same once they've been removed/refitted no matter how careful one is... Will get photos soon & will have a play with scrapyard sunroofs when the chance arises to see if it's possible.
And Jim, ol' friend, ol' buddy of mine

If you happen to have a diagram of how the parts all fit together or even just a pic of the parts in a dismantled state that would be most helpful. Off to take snaps and put the glass back on now...