IIRC there are two elements, the larger one being a fixed sum per unit volume. The other one is VAT, a variable (even if a fixed percentage).
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In fact the second link suggests I was wrong in that fuel duty only is 47 pence per litre. I thought it included the VAT element, so total tax is
47.1 p (to be precise) plus 17.5% of the total price. At GBP 1 per litre total tax is about 15 p VAT plus 47 p duty = 62 p so a lot closer to what Dodgy thought.
The planned increase in the duty has been postponed.
When I filled up yesterday it was 'only' 97 p at my nearest central London Shell station. :-(
DAS
For direct contact replace nospam with schmetterling