Very first thing to check : take it for a 20-30 mile run, driving perfectly normally. When you stop at the end, place your hand on all of the wheels, in turn. Are any of them particularly warm or hot ? This will tell you if the brakes are dragging, which is always a decent first culprit to rule out, and can make an amazing difference to fuel economy.

If not that, then it might well be something like the torque converter slipping, with it being an auto box. I'd be more inclined to look there rather than the DPF clobbering your fuel economy

That won't of course account for the difference in actual and indicated mpg. If any one knows the answer to this I would be very interested.

I have a Peugeot 406 and a C5 both with the same engine and transmission, 2.0. Hdi, the 406 is indicating an average 52mpg which is near enough correct when checking with brim to brim fill ups, the C5 is also indicating around 52mpg but in real life is only doing about 42mpg.

Any ideas on either count?

48,000 in 9 nine years is low mileage. The car should've clocked up at least twice that amount in that time. Low mileage most likely means short trips, unless the car was only used once a week. Short trips means cool engine, and cool engine means carbon build up. That leads to restrictions in the air intake, poor performance and poor fuel economy. Couple that with the fact it's an automatic it's not at all surprising that your fuel economy is poor.

That's a fair comment, but there is no restriction in performance. The car pulls really well and is really smooth in that respect.

I'd really like to get to the bottom of why the onboard computer thinks it's getting decent fuel economy when it's actually rubbish.

Cheers