Vauxhall Astra petrol consumption

i have terrible petrol consumption, does not matter whether i drop to 56mph, slow fast, motorway or b roads, i go through petrol like nobodys business, also used to have a strong smell of petrol or exaust fumes, eventually garage says they noticed a tube or pipe was missing ????? sometimes i still get the smell, especially at traffic lights or if just ticking over. but the main problem is the high petrol consumption, garage keeps saying they will do this and that, but i would like a straight answer from someone if they can provide me with a quick fix, is this a common problem with an 03 plate ? thanks