I got a tarpaulin from Tarpaulins Direct http://www.tarpaulinsdirect.co.uk/,
on the recommendation of others on this forum. £40 for an all over cover for my 4.8m, including wrapping round and under the outboard. Spent £20 or so on brass eyelets, trimmed the front edge to follow the shape of the tubes, then threaded a rope round the edge, pull it tight, and tie up at the back. (I keep the boat on a trailer.)
I know others have put more effort in stitching something more convincing, but I'm happy with my cheap and chearful approach. It has withstood some pretty strong winds.
Oh, I bought a white cover to avoid overheating too much in the summer.