I finally diagnosed the source of the rattle, or I should say my father did! He got the boot and I drove up and down the road slowly, he had it figured out within 20 seconds - the boot door.
When he pushed against the the boot door, pushing out if that makes sense, the rattling immediately stopped and it was quiet as a mouse. When he took the pressure off again - rattle, rattle, rattle. It's a plastic on plastic sounding rattle so I assume the plastic interior of the boot door is loose/poorly fitted/rubbing. The only access I have are the removable panels for the rear lights but I can't see anything there - there isn't much to see really.
Unfortunately it doesn't look like there's a DIY fix as I can't get any access inside the boot door, I assume there are specialist tools needed. Has anyone any experience with this before I send it into the garage and have to argue that it's covered under warranty?