Michael wasn't far off, but my problem was at the other end of the loom, near the battery. There is a black cover with about a zillon wires and connectors under it.
As soon as I removed the cover and touched the wires, the pump burst into life, so it's likely to be a poor connection among them. So at least I know where to poke a stick if it happens again.
Many thanks for all the replies, much appreciated
It's been our first effort at winter vanning, and despite our fears, it's been great. Lots of sunshine and frost. We aren't convinced if it will be as nice if it's cold, wet and muddy!!!