unusual idle

hi all . i have just received my car back from a mechanic after having the fuel filter replaced the oil and filter changed and the oil pressure switch . it a 56 plate vrs petrol remapped . it was fine before it went in nice steady 800 rpm never skipped a beat on tick over . now its idling between 800 -1200 revs the mechanic just said funny that i thought that when i test drove it .. it sounds like an air leak to me anything obvious i should be looking at that people have left off after having work done im quite mechanic savvy so im looking for a breather or air pipe split or left off ?