As Gingejbee says, 13p to 15p per mile is a typical cost for fuel.
The two travel survey companies that I work for pay 20p and 21p per mile though, to cover additional running costs.
The maximum that HMRC will allow as 'travel expenses' is 45p per mile.
You might also need to decide whether you want to add something in for your time. (One of the companies I contract for pays £7.20 per hour for travelling time, on top of the fuel allowance. I can negotiate an additional flat fee with the one company).
Driving at an average of 40 mph (allowing for built-up areas and winding country roads, which we've got a lot of around here!) you'd need to charge 18p per mile in order to achieve the National Minimum Wage of £7.20 per hour.
So, if you only want to charge for your petrol, I'd suggest 20p per mile for the return trip (which is, of course, the same as quoting 40p per mile in one direction). If you want to add in your time, I'd suggest 35p to 40p per mile for the return trip ( = a quote of 70p to 80p per mile in one direction).