But had a think overnight and decided that I could fabricate a repair if I could get home. Had another think, and made it home the 270 miles from Helmsley in Yorkshire with the help of a big screwdriver lashed to the side of the gearbox -

Was pleasantly surprised that this bodge actually worked - could select all five gears no problem!
Anyway, now thinking about a permanent repair. Going to try fabricating a piece from thick aluminium plate, and using a combination of a casing bolt and JB Weld to fix it to the gearbox case.
I would be hugely obliged if anyone who might have a suitable gearbox (Aircooled 1982 4-speed manual) in their workshop or yard could either take a photo of the box nose on, with a ruler or tape measure in the pic; or take accurate measurements from the centres of two of the casing bolts to the centre of the hole for the mounting pin, so i can get the hole for the new pin in the right place relative to the case.
If anyone has made a similar repair in the past, any advice gratefully received!