Oh one other thing that might be useful in general. I unbolted the EGR servo from the valve housing, took the pipe from the exhaust to the EGR valve off and played with the valve attitude. It is possible that the valve was sticky and this is consistent with the juddery performance at approx 2000 RPM.
What was interesting though was that when my mate pressed on the accelerator and loads of vrooming came out of the corrugated stainless EGR pipe I noticed that the EGR servo was moving in sympathy. Rule #1 in control theory debugging: if you have multiple potential points of failure in a control loop (which this is) break open the loop and chase the signals round until you get to something that looks wrong. Fix that then close the loop again to see if the fix was the only one necessary. Keep going.
Cleaned the EGR valve spindle and inlet port (sooty) with copious amounts of EGR cleaner hooch, the valve turns freely and the juddering is now gone...
Remember that in this situation the exhaust from the EGR corrugated stainless pipe to the EGR (disconnected in this test) plays straight onto those plastic fuel lines to the filter. You wouldn't want them to melt, now, would you...? Bend them out of the way.
.... but I'm still getting P0101 grr.....