I know there is previous advice on this issue but I thought I would add my experiences.

Recently the engine warning light came on. My simple plug in stated the EGR valve. I changed it about 2 years ago so with a mere 91000 (09 TiD auto) I was sure it was not a fatal problem.

I went down the route of 2 spray cans which reputedly may solve EGR issues. It did not work. I also tried a couple of fast runs and removed the EGR valve twice. On both removals and cleans no light for the journey but sure enough next day light back on.

As the car was running well I did not panic but tried half a tank of premium diesel. Erased the problem and 3 weeks later I have been clear of the dreaded engine warning light. So I guess the moral of the story is to choose the better fuel. That said I have used supermarket diesel in it for the last 5 years.