This company ranh me back and said that their Plastidip product is OK for Sunbrella they said that it is used in many marine applications such as painting anchors and anything that might corrode. The chap said that for fabrics it might be wise to use the primer to increase adhesion but it was not always required. He also said that the product had very good UV filters.
I'm going to try it as I've got nothing to lose except £450 + for some new covers, although it will be a while before I get around to doing it.
There is some technical info on the web site for those that understand it.
It is expensive stuff so I hope it works!