Whats the best way to clean my driver's seat? Its a W210 cloth seats, when I go mountain biking the mud off my back gets on the seat and makes it dirty.

Its elephant-grey, despite using seat covers the seat is pretty filthy - like a week old pair of jeans.

Any tips? I've tried my carpet cleaner (VAX) to little or no avail, and the covers are non-removeable as they're stitched into the cushioning.