Conrad, Nikwax do a range of products for exactly that. I wash my gear in Nikwax TechWash, a non-detergent washing liquid, but just to make sure I wash it again in Nikwax TX.Direct Wash-In Proofing as well (
both here). You have to dry it thoroughly in a tumble drier for best results though. My thick padded jacket takes an afternoon in the dryer! Works a treat though, my gear is clean, waterproof and breathable. Use it for my hiking gear as well.
Any walking/camping shop will sell it, though the Amazon link above is cheap.