Eh, I have the same problem, but I'll tell in litres, about 12 litres per 100 km...
But, I installed a gas system (prophan butan) in car, so now I drive for the price of diesel or even cheaper...
The bottle for gas is in the trunk instead of a spare wheel.
The car goes like on regular petrol, and the same MPG but double cheaper... Idon't know do you have that possibilities in UK.... It's a good sloution, I paid 320 euro for installing gas...