Big thanks to all writing on this thread - reading it helped me to spot that the problem with my
Core Light Black going blank was due to static and that I ought to return it to
Suunto.
Sent it via DHL, which was pretty painless, but hadn't heard anything from
Suunto after 10 days. Contacted them and they explained that the Light Black was out of stock (would have been nice to have been told without chasing) but that I could choose a different model if I preferred not to wait. Opted for a Regular Black (which I think I prefer) and it was on my wrist within 24hrs and has worked perfectly since (touch wood), even when I remove my fleece and hear the crackle of static.
Thanks again for all the help/support/info.