Are you actually self-employed? If "they" who pay you travel expenses are the only ones you work for, you need to consult hmrc leaflet ES/FS1, and www.hmrc.gov.uk/employment-status/index.htm#3
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If you qualify, you will need to complete tax return and sheet SA103S or SA103F, dependent on turnover. The notes say
"You may be able to calculate your motor expenses using a fixed rate for each business mile instead of actual motoring costs. If you use this method you must not claim capital allowances on the vehicle. You will find information on using mileage rates in Helpsheet 222 How to calculate your taxable profits."
Simply include payments received in takings, and claim the allowable amount in the appropriate box.