Brakes all free, oil changed, service not due for 8000 miles. Car starts and runs well but mpg dropped by approx 8 about 3 months ago and getting worse. New glow plugs fitted 2 years ago when I had this problem last and although many say these would not affect mpg, they did. No diesel leeks found. Glow plugs not checked yet

3 months ago would be october/november, that would be when winter diesel starts being used, I usually notice a 3-6mpg drop on winter diesel due to the "anti-freeze" components of the fuel making it less efficient. Also, warm up times are a LOT longer in this weather which might make a difference. Check tyre pressures as well, lower ambient temps mean lower pressures = more drag. Give it 2 or 3 months and it 'should' be back to normal.

How are you getting the 8 mpg drop figure, is that on a brim to brim to brim and maybe for 45 litres of diesel you get 80 miles less distance.

 

Is that short journeys where your oil never gets to operating temp or on longer trips.

As to the reduction in fuel efficiency with Winter Diesel, that is never shown to be actual if you are using winter diesel when the weather is no cold, 

like often during winter in parts of the UK.

Does the UK Haulage industry have to use 3%-5& more diesel October to April when there are above average temps, or even warm weather doing the same miles, carrying the same loads?

Have you perhaps changed tyres and the new brand is less economic?

Maybe the level of grip has a bearing too ?

 

Check operating temperature of the engine and not with the gauge on the dash. I have just had my thermostat changed as the temp on the gauge was dropping when in slow traffic or when stood (this started just before Xmas) I now realise it was probably faulty a lot longer before that (oil temp never got above 90 degrees on maxidot, now reaches approx. 100 degrees on a long run) and my average consumption on winter diesel is matching what I was getting in the summer.  

19 hours ago, davenG said:

l but mpg dropped by approx 8 about 3 months ago

 

By any chance have you had the EA189 "fix" applied ?