Oil and petrol

It seems that the price of oil has less and less to do with the price of petrol in the UK.
Recent falls in the price of oil have not only failed to be reflected in the price of petrol, but in fact seem to have pushed it higher!
And I am not talking about the ridiculous increases in "fuel duty" (read - tax), but the price charged by the petrol companies. Both before and after the recent fuel tax increase, it was noticeable that the base cost of petrol (before tax, fuel duty, what-have-you) increased despite falls in the price of oil.
I would really like to know how the big oil companies can justify this increase, as it seems to bear no relationship to a, the cost of oil and b, reality!