Yes I finally got it fixed by the garage.
They tried many things – unsuccessfully. Replaced the controller twice, replaced the throttle, even replaced the whole motor – with no success.
They liaised with head office in China, even taking some video footage, as the problem became more consistent and got worse.
Eventually, over 12 months later, the problem has been fixed.
Head Office suggested “replacing the wiring loom”. I don’t quite understand what this means, but it seems to have fixed the problem. All good since. Hope that helps.