Hi - I thought I'd just give you an update for anyone who's interested. My very kind neighbour (who is great at all things electrical and all-round practical chap) tested it with his multimeter and the reading for the charger was zero. It is an ex-charger.
Thinking it might be a break in the electrical cable, which is very thin, or the fuse in the plug, he opened up the plug and was confronted with, in his words, a black mess. Looks like some part in there has burnt out
I replaced the battery of this Dyson about a year ago with a non-genuine one - could this have caused the plug to burn out?
Anyway, looks like I will be buying a new charger for this from somewhere. Hope to get a few more years out of it, preferably without the house burning down :-/