Thanks for your input, the car has been in for one day and as with gardengnome’s reply, nothing was found and the car was said to be fine.
The figures below are taken on actual diesel used and mile travelled;
Bought car 3068 – current 4487 = 1419 miles
Litres used 226.70 = 49.28 Gal.
1419 miles ÷ 49.28 gal = 28.79MPG
The car’s computer is indicating about the same, give or take a mile or so, see below.
Fourth Fill up - 24.01.11
Diesel used - 44.65ltr = 9.71 Gal
Mileage covered - 280 = 28.84 mpg
computer says - 29.9mpg
What I find annoying is that I could have got a Renault Sport and still average over 35mpg?