Swapped a while ago to the more expensive diesels. Needs two or three fills to clear the old stuff and see the difference. Found I get 10-15% more mileage to the gallon and much better performance and acceleration than on normal diesel with my 1.6.
OH with his 2.5 litre gets about 20% more mileage, so well worth the extra at the pump. Generally use Shell, BP or Esso. Find a lot of the supermarket petrol stations don't sell it yet.
Local BP Station Ordinary diesel 135.9p Premium Diesel 143.9p
Price difference is only +6% approx, so well worth the extra at the pump.