I've just been out for another test with cleaned up plugs and the good news is that none of the cylinders are misfiring, at least not constantly so the plugs and coils are serviceable. Pulling each coil in turn resulted in a misfire which cleared immediately it was replaced although number 5 did hesitate a second or so coming back in.
I also did the screw driver against the head trick on all the injectors which were all ticking away nicely. The same trick on the chain tensioner also showed no nasty noises as I'd come to the conclusion that it could be a highly probably source of the rattling. Using the same trick I've narrowed to the source of the rattling sounds to the front of the head on the VANOS housing only including the solenoid. If you move to the block, the water pump housing or even the head directly behind the VANOS you just get a nice swirly noise of things rotating smoothly so it looks like I've found the culprit.
It may still just be a case of the car needing an Italian tune up down the dual carriageway but that's not going to happen until the spring post IVA so I'll pop the rocker cover and VANOS off over the winter and investigate further with a view to replacing the offending seals etc. There is always a chance that it's the top chain tensioner that is kaput but that is impossible to replace as far as I can tell without locking the cams (the tool for which I don't have).
As always, thanks for all the pointers.
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