Whens the last time it had new spark plugs and oil change? That can effect your fuel consumption if its not changed for its yearly service, to reset your ECU though you only have to disconnect your batterie over night.
Im thinking of doing this becasue mine is **** (approx 30 mpg all the time) and i am getting it serviced soon.
Alos think cos someone started modifying my car that they may have put one of those "£2 superchip resistor thingio". Becasue they make it run crap and swig the juice.
If anything a ECU reset will only make it worse (for the first 300-400miles at least). I'd check other things as T says, air filter and even simple things like tyre pressures .