Spoke to my client who informs me they have 3 HTC X1's in the building.

Sounds like a suspect - i'll be applying the firmware update.

I would have applied it sooner had I known the symptoms, but there we go

Here's hoping

Good luck dellboy, my fingers are crossed for you ;)

Alan.

Just a quick update - since firmware has been applied the symptoms appear to have now vanished, so it certainly seems that it was the mobiles causing this problem.

Continuing to monitor - but if anyone else gets the same symptoms and there's one of these phones in the area it may be worth applying the update

Thanks for all the comments