I think the total including VAT is now 65% of the cost of a litre however it still stands what I said (unless you have a massive increase in tax).
If petrol was say £1.00 per litre and 35p is the refined cost and 65p is tax then if the refined cost was half at 17.5p, the tax would only be 32.5p and hence the pump price would be half at 50p.
So from July 2008 to Dec 2009 when there was hardly any increase in tax yet the price of oil fell by half, you would expect to see mroe than 10% less at the pumps.
However, I will agree that over the years the amount of tax has grown and grown on a litre a fuel and is now at silly amounts. Every 1p increase due to the cost of crude going up means 1.65p on the pump price.
Doesn't quite work like that, there is (as posted) a 56.19 'duty' on fuel, and 17.5% VAT,
So using some of your figures:
If the price of the actual petrol was 35p (thats the crude oil, refining, profit to oil company, running costs of petrol station and profit for petrol station), then the end price would be (35 + 56.19) * 1.175, or 107.15p per litre.
If the petrol price halved to 17.5p, then the end price would be (17.5 + 56.19) * 1.175, or 83.52p per litre.
So a halving of the petrol price gives a ~22% reduction in the cost of petrol, given the above figures, also note that since the petrol has roughly halved we've had 2 or 3 raises to the duty (of around 2p a time iirc)
I really hope my maths is right there...