We have an 05 mk1 1.6 focus hatch...
This year we had the timing belt auxilary belt and the water pump changed as the car had done 68k..
I also the front anti roll bars changed, the rear drop links, new sparks and an oil n filter change...
Now ive notced we are doing less milage per fill up over this past few weeks...
We filled the car 10 days ago and its hoovering between 1/2 n 3/4 and we have done only 107miles its usually near to the3/4 mark....
There are no leaks as ive looked..
Any ideas..

 

One thing that could affect it is the weather, as it's getting colder the air is more dense than normal so your ECU is providing more fuel to compensate for the extra oxygen.