Well I'm pleased to report that I think the problem was no oil in the filler bottle.
Don't know why but I have filled it, warmed the engine, waited 5 minutes and checked. Needed a little more but no light on dashboard - thank goodness as I'm off to Le Mans on Saturday - and all gears are selecting OK.
Will keep you all posted after the weekend.
PS checked the sensor on rear engine cover and cleaned it just to be safe.