I know people keep harping on its only 1p or 2p etc, but in the last year its costing me £15 more to fill up!
Shell garage by me has gone up ~5p in a month, so if it goes up even more I'll be peeved!
£15 more a what? Day? Month? Year? Are you driving exactly the same amount as last year? Same route? Same car? Same level of acceleration?
If oil goes back over $100 and towards $150 again, we're ******.
And that shows the smoothing effect of having a fixed cost of tax. If the tax was a percentage of the fuel price, with fluctuations like that (and even more over the very short term, but just not visible on that graph), then petrol prices would be a lottery. You could pay £90 to fill up one week, and then £60 the next.

No it does not, a smoothing effect would be to keep prices constant by varying the tax.
They're hardly likely to vary the tax day by day, week by week though are they.
Hey, you can pay £1.20 for a loaf of bread these days too.

A petroleum company knows what petrol is going to cost at the pumps several weeks in advance since it is based on oil prices from whenever they bought oil. As such the government can know weeks in advance what the petrol will be.