There's a motorhome dealers not far from the start of the M27 at Portsmouth, called Southdown Motorhomes. They have a good accessory shop and their parts manager is very helpful. It might be worth giving them a call. They were very helpful when we needed parts for our caravan.
i live in Norfolk near to the caravan accessory shop in West Runton so i am a regular customer for all the bits and pieces we all need from time to time...last Sunday went down to buy a new water connector and some toilet chemicals and a small length of awning skirt to be told we are too busy to cut the skirt for you today !!!! so went home ordered online (at £1 cheaper )and delivered Tuesday.... bibi caravan accessory shop and hello ebay
I was going to advise there was a tow sure in Southampton but I just read it had closed. However I bought a fly screen for the small roof light in my bailey ranger off eBay for £11. It's actually cheaper to buy one from the bailey parts store web site but they were out of stock until after my summer holiday. Just look for MPK fly screen. In any case I have not seen one in tow sure or caravanstuff4you which has a shop in Hanley swan near me. My fly screen wouldn't stay closed because a catch had broken off it.