fuel consumption

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HI I,m posting this topic again as I,ve been away and not had a chance to catch up on previous replies. sorry to anyone who has felt thier advice has fallen on deaf ears.
THE PROBLEM-
I own a daihatsu sirion 1.3s 2005, after a recent MOT and service the feul consumption has increased from about 39mpg to about 25mpg or a full tank will give me about 200 miles.
The mechanic replaced the spark plugs with NGK IFR6J11 ones,during the service.
I have been back several times with the car, had dianostics, and four gas tests done with no apparent problems, no obvious fuel leakage, and no sticking brakes.
I have been told by the mechanic that driving with the A/C on is the problem,not getting the engine temp up to optimum could be the problem.
I,m at my wits end and thinking of getting a second opinion from another garage so.
1/ Can anyone recomend a decent garge/mechanic in the Glasgow area
2/What can I do myself ie checking for fuel leaks, sticky brakes,etc.
3/could it be the spark plugs,I,ve checked the NGK web site and the recomended plug up to 2005 for Sirions is different NGK BKUR6ETB-10.
Any help would be much appreciated
thanks very frustrated1!!!