Latest responses from the dealer:
Talked to the previous owner at length last week, he said he has no idea what it could be, the cover was on all winter, he took it off when he towed it to our yard ,we didn't fit the cover, it stayed rolled up in the aft of the rib where you found it. we were just the agent for the previous owner, we charged him £150 commission to cover the advertising cost . he isn't prepared to offer any compensation. so not much I can do. I can forward a bottle of ribshine which is a strong rib cleaner, it normally works very well.
I refused this offer of free Ribshine and got this response from the dealer:
I guess you had better get some advice then and start proceedings.
Look forward to hearing from you.