Thanks. Turns out it was the pins in the top of the cyclone. They are quite fragile and easily bent out of alignment with the socket on the motor unit. Once I'd straightened them, all was well. Which was good, since the cyclone and motor assemblies are only sold as one £60 unit, despite the cyclone being a 'user serviceable' part, as defined by Dyson, since it has Philips screws, not torx. Thank you Dyson.
While I'm moaning, the cyclone design on these is, IMHO, seriously flawed. It traps even fine dirt at the top of the cyclone, where it clogs and eventually triggers the blockage sensor. I spent a couple of hours trying to pick out the worst of the gunk with various implements, including a narrow bottle brush. Normally this would take 5 minutes with a cold soak and then a hot rinse under the tap, but since Dyson have in their infinite wisdom, chosen to run the wiring internally within one of the cyclone channels, total immersion is not a viable option.
Since the bottle brush etc still proved inadequate, eventually I resorted to careful use of the hot tap while trying to avoid soaking the wired channel, which cleared it sufficiently to satisfy the blockage detector.
Definitely a step backwards from the V6, 7 and 8 in this respect.