Dan,
Don't forget that the 2.5% reduction is from the cost of the fuel+duty+delivery/retailer not what you pay per litre.
I mean if you pay £1 for a litre of fuel that includes VAT so the actual cost of the fuel, inc duty etc was 85.1p so the new cost with 15% vat would be 97.9p i.e. only 2p less.
Remember the Fuel is only about 32% of the price, The retailer/delivery accounts for 9% and then the Gov't tax it twice with 44% Duty followed by VAT on the lot at 15%.
So if you are paying 88.9ppl that is:
Duty 39.1p
Fuel 28.5p
Retailer 8p
VAT 13.3p
When the price of fuel goes up and down its that element that moves so with Duty going up the cost of fuel must have dropped quite a bit for the overall cost to drop.
I think?