After three trips to the main dealer they have finally diagnosed the source of the blasted rattle!
The latch the the boot locks on to (the vertical steel latch in the tailgate) is loose. It's a non-movable, non-wearing part and the dealer has never seen it before. They're going to replace the latch, it's seat and the the trim on the tailgate as it's been removed several times at this stage, all under warranty. New parts arrive next week

Although it's taken some time I have to credit the main dealer. They were very easy to deal with and at no stage tried to fob me off when I was complaining of the rattle. Looking forward to some considerably quieter motoring
