The 1 litre should be giving around 40 mpg, if you've had it serviced and everything was fine then there should be no problems, what milage has the car done ? does it run and tick over smoothly ? it hasn't had one of those useless 'add 20 bhp' ebay resistors fitted has it ? are there any other problems with it ?
The diagnostic socket is next to the fusebox under the fusebox cover by your right knee when sitting in the car, I'm not aware of any free software that would allow you to plug your laptop into it, but there is more knowledgeable members on that side of it than me.
