I have the 1 litre ecoboost fiesta but the stop start function doesn't work.

Also fuel economy is pretty bad. I got 42 per gallon last fill up and I don't know why. I don't drive like a nutter.

Any suggestions would be greatly appreciated. 

42 mpg is not horrendous. I use to get about 47-48 from normal unleaded and most of my driving is a steady 60mph on M/ways

Assuming you definitely have start stop fitted (you should have a button somewhere which will temporarily disable it if you have). Then the battery monitoring system may need recalibrating. Basically the system thinks the battery is flatter than it is and won't risk not being able to start the engine again.

Kudos should really be to @iantt for the procedure but I can't find the thread again (sorry Ian!).

Edit: Ian, please feel free to link to it in reply so thanks can go to the right place =]

Turn the ignition on (but not the engine).

Wait 10 seconds.

Press the rear fog light button 5 times.

Press the hazard lights button 3 times.

Wait 10 seconds.

The battery light on the dash will flash to show completion. Just drive normally after that and after a couple of miles it should work.

If not, your battery may be heading poo-wards. Test the voltage at the battery itself with a multimeter with the engine off. Anything less than 13v isn't ideal. Anything less than 12v is usually a flat or knackered battery.

S/S may also struggle to engage if you have a lot of draw - air-con, heated windows/seats etc.