I have a 2001 Fabia Comfort SDi which has 93k on the clock and runs like a dream. In the last 6 months or so I have noticed quite a reduction in the fuel economy. Whereas I could expect an ave mpg of 60+ when travelling on the motorway at 70-75mph, now I am lucky to achieve 55mpg. I have changed the fuel, air and oil filters and changed the oil but with no improvement. Obviously I can expect a reduction in winter due to denser air and using the heater etc but not this much. Can anyone advise?

Actually, you can expect that sort of drop in fuel economy between using the heater more, and "Winter diesel", which just doesn't give the same mpg, largely due to the "anti-freeze" additives in it.

If it helps I get 60 on short runs and 70 in summer on long journeys.

I have just checked recently , and got a reading of 56. I would put it down to Winter .