Simsy - yes, because although I buy fuel in litres, I understand mpg, and all fuel consumption seems to be quoted in mpg rather than mpl. Thanks for the link, but I want to be able to enter the relvant details each time I fill up. Sad, I know!
Belatucadrus - thanks, I think that's what I thought I meant!
I have an Excel spreadsheet that does the mpg for me as well as totalling the costs of petrol and the average mpg over the period of time that the sheet refers to. If you would like a copy of that, I will gladly send you a blank copy for you to do with what you will!