The price of crude oil on the world markets is now around $32US, the current exchange rate between the US dollar and GB pound is $1 = £0.68p. A barrel of crude oil is 159ltrs.
So $32 x £0.68p = £21.76 and £21.76 ÷ 159ltrs = 0.14 pence per litre, both diesel and petrol at Asda's are now the same price 0.997 per ltr....... even if the refinery costs and profit margins take it to 20p per ltr the Government are raking in nearly 500% tax!!
I thought that would cheer you up