It is controlled by a two-sided level which operates (what I now know) is a potentiometer.
The lever is mechanically held (by a 2-way coil spring) in the centre position. One way is forward, the other reverse. The degree of movement from centre is the speed.
Last couple of occasions, the "reverse" operation hasn't worked as expected. I managed to get it to work by a "whack" motion on the lever, but SWMBO couldn't seem to work it.
Cue getting back, me removing the complete tiller/motor/batter assembly, and removing the cowling to reveal the top innards. Nothing obviously amiss.
I was suspecting a dirty pot track. However once dissembled (you can see where this is going) the reverse movement worked fine. Which continued, now the thing is reassembled. Been whizzing around for 15 minutes back/ forward back/forward. No hint of a problem.
My plan is to carry on again until it either does it again, or we scrap it in 20 years time with it never having caused a problem again.
I'm not a great believer in problems spontaneously fixing themselves. My guess is still a dirty pot track ...
If nothing else I took photos while it was in pieces for serial/part numbers. I am not paying £400 for a whole control assembly if a £1.00 pot would fix it.