Whizzy hardware whizzes after all. Go me!
Go Steve, more specifically, since he correctly diagnosed the problem: it hadn't occurred to me that generic mains adapters sold in Maplin came with several different power capacities, and I'd inadvertently bought one capable of supplying perhaps 30% of the power the whizzy device actually needed. Took it round to Greenend and ran it on one of Steve's power supplies and it worked fine. So, that'll be a trip back to Maplin to sort out the misunderstanding and then all should be cool.
This afternoon I got to look round ARM's new office building, which my group will probably have moved into once I get back from my extended holiday in October. Seems a bit odd; lots of strangely shaped rooms, glass walls, and so many non-right-angles it almost felt like being back in a Cambridge college. Evidently office designers have decided that "boring and functional" is so 20th century.