With their price drops, roll-backs, brand matches - as well as that old firm favourite, the two-for-one offer - our leading supermarkets are doing battle for our cash. They claim their price war is good news for shoppers in these tough times, but are... (more) their money-saving offers all they seem? Sophie Raworth takes her trolley round the aisles of Britain's biggest supermarket chains and reveals some nasty surprises at the checkout.