2004 Volkswagen Passat Lack of power

I have a 2004 Passat 1.9 Tdi (130 bhp model), It could be a coincidence but shortly after a basic service (oil and filter, air filter and fuel filter) the engine all a sudden became very sluggish. Its starts and idles as normal and the engine check light is not illuminated. It seems to have very little power untill approx 4000rpm when cold. However once it warms up to normal operating temp it doesn't seem to have any power at all. To maintain 70mph in forth gear requires the pedal to the floor and takes an age to build up that speed.