Many thanks all for your responses. Being a bit of a cheapskate, whilst I entirely understand the attraction of the synthetic materials, I will probably go for something I can fit myself.
6mm teak including the caulking would come in at about £1200. Looking at alternative timbers, the most promising I have found so far is a processed timber called Kebony. It has a density and movement with moisture characteristics similar to teak but is much cheaper, perhaps £300 including the caulking material. I am told that the pontoons in Yarmouth IOW were recently re-laid using it.
I have made up a trial panel which is nailed to the front of my shed at home and we shall see how it looks in a few months' time.
Martin