This might be a bit late, but I also needed a replacement screen for a 4m Searider.
I called Avon and they were really helpful and sent me the original spec drawing for the screen (still in Imperial measurements). While I could have purchased a sheet of polycarbonate and had a go at cutting it myself, I found a company on the south coast (near Portsmouth) that can make one for £146 + VAT. Email email@example.com
if you need one. If you wanted more than one, the second one would be around £95 + VAT - ideal if you have a friend that also needs one.
I have attached the Avon drawings if anyone wants them.