I didn't try adjusting anything, by the roadside. It all 'looked ok', i.e. nothing had come loose. (That's why I went for the bit that I couldn't see – an internal inspection of the selector mech including the spring.)
I will go through all the adjustments though, when I get some time, as something must have gone off a bit.
Ever since this incident, I have been wondering again about your comments – that the box shifts better on the race track. I was particularly thinking about it because the conditions that I was in when it wouldn't work were the direct opposite of racetrack conditions: very little cooling airflow, slow riding with gears chopping up and down in second and third for miles on end...