Sam if it is breathable (which it probably is) you will need a non detergent powder for breathable fabric, you can find them in all good outward bound shops. That alone should do it but as 'belt and braces' you could get the re-proofer by the same manufacturer
Hope this helps
i use a product called NIKWAX Tech Wash to wash the jacket and trousers and then I do a seperate wash with NIKWAX TX Direct Wash in (which is the waterproofer). You then don't need to waterproof the clothes for another 6 or so washes. You can get the waterproofing stuff from any good ski shop or ebay.
Hope this helps.
I wash mine as normal and then a week or so before my next trip I treat them with 'Fabsil' it's a waterproofing agent which you spray on so is really simple to use. You can get it from most outward bound/army surplus stores. I think it's about £6 for a large can which does about 4 ski suits.