hmmm ... that was short lived!! .... but it looks like now another problem fixed ... hopefully :-/
I removed the motor & gearbox assembly (2 screws and a 8mm hex bolt). Unscrewed the controller (3 torx screws) and then the gearbox cover (1 more torx screw). Inside the grease had congealed. I removed the motor, the gear that drives the sunroof was very stiff - no wonder the controller thought the roof was jammed!
Cleaned up the inside and re-lubricated pretty much all the gears etc. with white lithium grease - sounds sooooo much better. Refitted to sunroof, which now slides back & tilts without jamming ...
The only "gotcha" I found was that on re-assembly you need to close the roof (using a screwdriver if necessary), then reconnect the motor assembly and slide/close/tilt/close it first. THEN refit it to the sunroof - otherwise I found that what it decided was the closed position was in fact half-open on tilt ... so far it looks like job's a good 'un