Brew,
35-40 on that sort of trip is very poor for a diesel. I'd expect about 45-50 with my 1.2 TSi petrol.
It seems to be characteristic of the 1.6 CR diesel that there are big variations in mpg from car to car though nobody seems to know quite why. There was a guy on here (can't recall the name) who eventually managed to return the car to SEAT and get a refund of the purchase price!
Let's hope that there is a simple explanation for the trouble with your car.