25mpg is a little low, I had the same car some years ago and it was averaging 30/35mpg.
Could be a whole multitude of things. Does the car lack power at all? does it misfire? what colour is the inside of the tailpipe of the exhaust, if its black and sooty, it's a tell tale that it is overfuelling, Sadly nothing to adjust however as it is all controlled by the ECU
Problem could be down to a number of things. Faulty Engine management components or sensors, worn throttle body etc. Sadly there is no Engine management light to tell you when there is a problem.So You could spend a fortune having all sorts of tests done for what? an extra 5mpg.
I'd put "her" out to pasture now, I'm sure she doesn't owe you anything plus a 11 year old mondeo isn't cheap to MOT either
CJ