I had this after using a different screenwash from my usual. I think the two somehow reacted and it smelt terrible. I had a long summer journey to go on, so I filled the reservoir up with plain tap water and used it a lot until it ran out (for some reason, the back squirter stops working before the front...) and then refilled it a couple of times at service stations. It stopped smelling after the second tank-ful. Now I only use one sort of screenwash and I haven't had the problem again. : P