Came back today repaired. Allegedly.
Unfortunately it won't respond to the remote control. I thought there might be a fix for this, e.g. it might have been reset to the wrong channel or something, but apparently not. When I phoned I was told "Oh, we have to disconnect the sensors to get at the motherboard, but I can't believe I didn't reconnect either of them."
Needless to say it has to go back to fix this, if I try diving inside the warranty goes phut. The cleaning instructions for this (ceiling mounted) projector, incidentally, say that it has to have the dust filters cleaned every hundred hours or so of use. For us that could easily be more than once a week if I take them literally. Sometimes I despair...
Don't get me wrong, I think that they're a great idea, especially combined with VCR / DVD and interactive whiteboards. It's just that I suspect the technology still needs to get a LOT more reliable, and silly things like this don't help to convince me that it's got there yet.