Several possibilities (I am moored in Royal Clarence despite the hike in fees this year...). There is a new company on site just opposite the marina offices called Covers (just at the entrance of the big car park), they might be worth a try. I have usually taken my covers to Hardway Marine Chandlers (Hardway), they have a valetting service and will usually check things over too. Hardway's number is 0239 258 0420. Ask for James (he does sails and rigging) and he also has his boat in the marina at the moment so he might be happy to pick the cover up for you.
If you speak with the Tony, Jane or Mark at the marina, they will usually be able to help you as well.
Always used to use the pressure washer on the covers laid flat on the lawn. Never seemed to do any harm as long as I didnt go silly and get the wand too close. try at your own risk.