A slight rattle when on cold start turned out to be the tailpipe steady bracket returns which had snapped. They are held to the inlet pipe which runs into the tailbox by a couple of securing brackets and having snapped just chattered annoyingly. More so when on cold start, it just made a 911 sound cheap!
So the tail trims were removed (a single clamp each side) leaving the tailboxes on the car and the snapped return fittings were neatly welded back on. The image below shows the snapped brackets arrowed red and the welds, arrowed white.
Once refitted no more annoying rattles. At the same time it seemed rude not to polish the trims themselves using Britemax Metal Twins.