[ziggi]

You can claim the difference (ie 40-13p) from your tax return each year. Goto the Inland Revenue to find out how to get a tax rebate on this. There's calculations and a table for petrol rates etc. All you need to do is list all the journeys, the distance, the rate you got paid and the difference (on a spreadsheet).

I do this every year. In fact I think you can back date it upto 6 years past so if you've got all the records you can claim the difference back.