I'm pleased to report that one clockwise turn (ish) on the linkage (shortening the effective length) seems to have restored my 4 speed and reverse gearbox to 5 speeds and reverse; at least statically selecting the gears. I haven't got time to take it for a road test, as I'm flying back from France tomorrow and I'm in shutdown/lockdown mode.
So thanks everybody - for once it was a simple job. Would that they were all like that...