Vauxhall Meriva very heavy on petrol

I recently bought a brand new Vauxhall Meriva (1.4 turbo exclusive)and despite numerous checks I have been unable to get more than 32mpg, there is also a smell of petrol coming from under the car when you stop.I've had the car back to the dealers on three seperate occasions only to be told that no fault can be found.I've now had to refer the problem to Trading Standards as going back to dealer seems a total waste of time.