Well, I went over all the connections with a fine toothed comb this morning. Everything prefect. Took out the PCV valve and checked it rattles, then just as I was putting it back, I spied something inside the cam cover. Fished out two large plastic pieces of what looks the remains of a plug boot. My diagnostic skills hadn't accounted for that, lol.
I then bridged TEN and GND and reset the idle and everything is back to normal. The plastic bits were too big to fit through the PCV and get into the throttle body thankfully, but they must have been blocking amy EGR recirculating. I'm not too impressed a previous owner has left them bouncing round the cam cover either
Time for a new gasket and proper clean out for the next job I think 
Thanks for the help Brucey, thinking it was a vacuum problem put me on the right track
