I looked it up in my recently acquired book Field Guide to the Moths of Great Britain and Ireland. The drawings are by Richard Lewington and I can't praise them and the book enough. I thoroughly recommend it.
The Vapourer moth is apparently very common though this is the first one I've ever seen. I guess I haven't been looking for moths that long! This is a male as the female has only rudimentary wings and is flightless and is a light grey-brown in colour.
As this moth was resting on my window I was able to get a nice photo of its underside as well.