John Lewis sells white organic Fairtrade sheets, though I am not sure they have duvet covers. I have the fitted sheet and it's nice quality and well-priced though: www.johnlewis.com/john-lewis-fairtrade-organic-cotton-fitted-sheets/p119...

They also sell a flat sheets www.johnlewis.com/john-lewis-faitrade-organic-cotton-flat-sheets-white/p... and pillow cases http://www.johnlewis.com/john-lewis-fairtrade-organic-cotton-pillowcases...

Hope this helps.

I discovered that Gudrun Sjoden also sell lovely organic cotton bed linen in their homeware range. http://www.gudrunsjoden.com/uk/2013-homesummer/category/bedroom.

The signature floral print duvet covers come in two colour ways and are reversible (with a different design on each side). They don't sell sheets so would work well with the John Lewis ones in my post above.

Try Luma and also Liv uk. beautiful cotton bedding.

here is also a superb source for all kind of bedding products: You can find here beautiful range of bedding products: like bed linen, quilt covers , pillowcases e.t.c http://www.beddingsquare.com.au/

I've had nice things from these people, too :
http://www.fairlycovered.com/index.php