After three years' ownership, I finally solved the boot rattle this morning. It was the reversing light unit.
I read on here someone replaced the seal on a light unit to stop their rattle, so took it off to examine the possibilities and noticed it rattled. I couldn't see why it rattled or get into it to fix it, but it now seemed likely I'd found what has been driving me nuts. I found a replacement unit on ebay, it was delivered this morning and the rattle disappeared as soon as I put the new light unit in. Three years, miles of tape on anything that wasn't nailed down, new gas struts and lots and lots of swearing, but at last it's gone! No doubt another one will appear, but these small victories keep you going.