The British consumer is no stranger to the ultra-competitiveness of the nation's big retailers. Over the past decade we've seen them go to war over selling baked beans for less than it costs to produce them, telling farmers to provide food at cost price and battling for supremacy over the high street. This week comes a further, unexpected twist. The latest trading battleground is over who has the shiniest ethical halo. On the eve of Britain's 14th Fairtrade Fortnight, which starts tomorrow, Marks & Spencer and Sainsbury's are launching products that - to borrow a sharing, caring cliche - 'give something back'.