Just back in from the garage and really grateful to see so many comments (some of which may be above my skill level!). I don't think the clutch is slipping, no other signs of this when I'm on the open road and I live in a hilly area! The gear lever doesn't pop back into neutral. The one item that, for me is from left-field, is the OD. Certainly, the Canley notes suggest this is a possibility that I had never considered.
Time to have a look through their guide notes. Ohh and I got it to do it again this afternoon. Into reverse backing happily into the garage, then on the small slope into the doorway, no drive and plenty of revs; into first forward 10 yards and then back into reverse - no problem! Got to love a weird intermittent fault one to keep me going in the cold dark evenings.
All other thoughts welcome and thank you
Regards
Graham