I would base your calculation on the mileage rather than the duration of the journey as it could be the best part of 200 miles on clear motorways, 50 miles on single-track local roads or, as I would suspect, some kind of mixture of slow and fast roads.
You can easily find out the approximate mileage online with Google Maps or AA Routefinder, even if you only know the name of the nearest village or town.
It also depends on what car (size/engine size/model/age) it is as to how much the petrol will cost. I think standard business mileage is 45p, but that takes account of more than just the petrol costs (depreciation etc.).
Assuming it's a newish, medium, mundane family car, I'd personally reckon on about 25p per mile and then divide the total between the number of people (or at least adults) travelling. If anything, I'd round up a bit rather than down, to take account of the other car running costs that will be borne by the driver, which aren't as quantifiable as fuel (repairs, servicing, new cam-belts etc.), but which still tend to increase the more miles are driven.
Hope that helps as a starting point - others may well have differing suggestions 