I don't know if this will help you in the UK, but I made a frame using metal electrical conduit. You can find the ridge pole fittings on the web to provide for the athwartship braces. I went with white "Ultra Kover" material which was fabricated to exactly my size specifications. This cover has survived 4 winters in Maine, and the cost was very reasonable (here again - cost in US). To secure the cover, I used the "Kover Klips" from the same source. These clips function on a dovetail principle, and they are fabulous!
Here is the link: Kover Klamps - Kover Klip Video
No connection to this company - just a satisfied customer. Oh - and I can't stand shrink wrap - it is an ecological disaster, and it only gets used once!