Ah yes, upon reading more carefully I think the ones that can easily go in the machine are the modern waterproofs. I have now read of people hosing them down in the summer, which might work, or as you say sponging them.
I've looked up Sally's suggestion of Nikwax, and their product chooser does direct me to something called Tech Wash, which isn't too expensive. I can't find if anyone has used it though, I'll have to have a look next time we are in Scats or somewhere similar. I don't mind buying a couple of products in this instance as it might mean I wind up with a coat significantly better than any I would be likely to lay money out for...