Car has done 50000 miles
It now has lost its ability to rev much above 3000 revs, even down hill. Given time, a long time, it will make 4000 revs. It has lost its sparkle low down the rev range.
It seems that the more load you put on the engine, i.e taller gear the less it wants to rev.
No warning lights on.
Car averages 55mpg. over 60 on motorways.
New diesel and air filters fitted with no effect.

Anyone had a similar problem and managed to cure it.