Try the Automobile Association (AA), www.theaa.co.uk.
I am not sure whether they will cover a non-british bike, but it's worth a try anyway.
By the way, the carnet de passage is a nightmare by all acounts: in itself it is only about £70, but aparently you need to have a deposit for twice the value of your vehicle in the bank.
I am planning to go to Mali in november, and I think the carnet the passage is not necessary there, but do you know if I need something else? (I have heard about the carte grisse, but I do not know what is it, or how much it costs, or where to get it) (my e-mail is email@example.com