On Fulbourne we had always used black canvas from which we tended to get about 5 years useful life.
The last time we went to replace them we could not find anyone supplying black canvas for love nor money although I believe that you can still get green ones.
In the end we went down the synthetic route and used a material called Regentex. We got it made up by a black country firm call DH load Controls, details here
I believe that Kildare has cloths made from the same material, bothboats should be at the Braunston event if you want to check out the material.
Tim