Yes, it would be interesting to measure the wall thickness of the seat tube.
Anyway, by way of an update:
I've been in touch with Hot Wheels, the Warranty Manager there has been more than helpful. He has a Zaskar reissue himself and said it's the first one he's dealt with in the UK that has developed a fault, but did also venture that he doesn't know how many of them are actually being ridden...
Anyway, the upshot it they still have a couple of Medium frames in stock, I sent him a load of photos and he seems pretty sure it's a warranty issue. We're just negotiating whether I have to send the whole bike to them or they'll do an advance replacement - I must admit I'd rather not have to have it couriered down to Poole, although they're offering to rebuild the whole bike onto a replacement frame - mind you I'm quite capable of doing that myself! At least they're making positive noises, which is more than I expected to be honest.
Phil.