Not sure if this is useful to anyone but I've created an excel spreadsheet which takes 4 inputs (date, fuel cost, fuel price and mileage reading) and works out:
Miles Per Day
Fuel litres
Gallons
MPG
MPL
Remaining L (when you filled up)
Miles Remaining (when you filled up)
Pence Per Mile
Miles Per Tank
and also averages most of the above, from the averages it works out what your flashing and solid light reserves should be and when your next service should be due.
Yes I know its excessively geeky and probably I'm the only one interested enough but just in case I thought I'd put it up here.
Sadly you need office 2007 to make proper use of the averages, it might work with the office 2007 filters available from Microsoft but I'm not sure and can't test that at the moment.
Here's the link if anyone wants to try it out, its populated with sample data to give you an idea of how it works.
http://www.brettnet.co.uk/gallery2/T.../fuelcalc.xlsx