Jeff Layman wrote:

It's two single receivers.

I'd look at what is near(er) to the one that's been unresponsive that
might interfere with it, can't see the transmitter getting different
interference depending which ID it's sending.

Apart from those two occasions I mentioned,
both systems have worked faultlessly for over 9 months.

I suppose one thing I could try if it happens again is to change the
pairing so that the currentÂ* "ceiling" transmitter is paired with the
"wall" receiver, and vice-versa. If the fault changes to the wall
lights, then it's the transmitter playing up, but if it stays with the
ceiling light it'll be the receiver. It's just that to do that means
crawling around the loft and accessing the receivers which are in an
awkward place.

That was sounding like a good plan, until the last sentence ..