Double insulated cooker hood with the usual plastic cased motor is
tripping the RCD it's connected to, but only intermittently and only
when changing the fan speed.

Thing I don't understand is how? Arcing maybe?
I did find a couple of discussion threads, but they were more about new
installations. This has worked fine for the last ten years*...

It's definitely not earthed (dire warnings against doing so in the
manual) and there is no measurable DC resistance from any metal parts of
the unit to mains earth. There's no DC resistance from L OR N of the
mains plug to any metal parts either - in case any one asks

* At ten years old I'm more than happy to swap it out, if it really is
the cause of the problem.