Thanks for all the help as above.
I've tried the seats folded down, plus rear belts plugged in, and adjusting the parcel shelf strings, but the noise remains!
I think I have tracked the rattling down to the rear air vents - one of these is missing the chrome adjuster tab, and am wondering if this is knocking around inside..
Can anyone shed any light on removing the rear vents/getting inside them?
Thanks in advance!