DRY STORAGE BAGS, £9.95 on eBay!
 

check these out, they are the Canoeists, Hikers, Etc type dry storage bags
ebay item number 150155805280 the eBay sellers shop is called Camping-Direct price is £9.95 plus £2.50 postage ( £1.50 per additional bag, UK, Contact for Overseas Delivery)
These seem quite cheap and I have seen other bikers using them as they keep ALL the water out of your dry clothes, sleeping bag etc.