Anyhoo, spent several hours taking the rear seats and trim out, removing seat belts, speakers, spare tire, rear demister/s etc.... along with taking a couple of passengers on 2 separate rides to see if they could help me find the source of the noise.
Finally found out what was causing it tonight. A screw in the ceiling light unit had worked itself loose somehow and was rolling around in the light unit whenever changing direction quickly!

Thought it might be useful to post this in-case anybody else has this happen to them, as it might save them using magnetic twisty implements in the dark recesses of the cars bodywork!
