Hi Bernie,
Thanks to your advice I’ve discovered that problem was not in the gearbox (thank goodness) but in the carburettor adjustment. I enriched the mixture, increased the tickover, and then ran the car on the M5 and M42. Problem solved! The engine must have been cutting out on the overrun. Your observations – measuring the play on the tyre wall – experience with your friend’s ignition – helped me to “think outside the box”(sorry!). I’m very grateful. If we ever meet you will find many drinks behind the bar with your name on them.
Regards
Bob