Well having the seats down and the parcel shelf out didn't fix it, but I think it has enabled me to locate the source of the issue.
The rattle was coming from the near side rear of the boot, where the first aid kit / warning triangle go. Been sat in the boot now for 10 minutes, and that section was very loose. Looking a bit closer, there is a clip behind the bottom plastic section of the boot (where the boot lock bracket sits) and that has come out, enabling the carpeted section to move about.
Put that back in, and it doesn't move at all now. So hopefully....that should have sorted it.