2000 Peugeot 206 high fuel consumption and exhaust

Engine Performance problem
2000 Peugeot 206 Front Wheel Drive Manual 39000 miles

couple of months ago had new back box put on, still sound like its blowing and liquid dripping from near where the box must join the rest of exhaust so took it back and was told "Yes it does sound like it blowing abit but its because new exhausts have built in blow hole and its fine? Is this the case?
Also I am only getting 250-280 mile out of a tank? Costing aroun £55 to fill. Does this sound write?