MAF (Mass Air Flow Sensor) sounds your other problem which often can go at not particularly high mileages.
Is the garage using a T4 diagnostic Computer as if not a Ex Rover dealer or a XPart Euro Service centre chances are they are using a Generic code reader not very applicable to our cars I'm afraid.
If your car has two pumps (one under bonnet) then starting problems may occur if in tank or under-bonnet have gone or on way out.
If you are failing to start with less than a quarter of a tank of fuel sounds like in tank pump.
No Orange Clip as far as I am aware on Diesels. (but knowing me could well be wrong)
MPG depends a lot on Driving style on distances driven.
With a Auto Box around towns you will be getting high 20's but once on a run or motorway driving you will be high 40's.
Many get higher but combined anything from 35 to 50 MPG.
Until a Diesel warms up it does drink more fuel as it won't use top gear,
so short stop/start runs or under 7 miles or so will use more fuel,
this will be noticed more as the winter arrives.
What are you getting MPG and use Brim to Brim method don't rely on on board Computer as can be inaccurate unless calibrated.
Welcome to the Club by the way.