Sorry, I ought to have clarified:
Unburnt hydrocarbons should have been below 200 ppm but were in the low thousands. Since posting this yesterday I took the car for an "enthusiastic" run along the A1 and can confirm the following:
1/ This car moves like a scalded cat when pushed! I LLOOVVEE driving it!
2/ At the commencement of the trip M.P.G. was at 43.1 and by the end of this high-speed run was 43.2, so in spite of being belted it's still pretty economic on petrol.
3/ Before the above my wife had noticed an occasional "engine management" symbol flash; examination produced a PO304 error, cylinder 4 misfire. I cleared the code and it DIDN't reappear during the 40 mile belt through the Hertfordshire countryside.
4/ I'm going to buy some injection cleaner tomorrow and use as per instructions.
5/ Would cleaning the E.G.R. valve help? I'm perfectly capable of so doing, but don't want to do so unnecessarily.