Hi,
I would recommend the new organic cot mattresses that have been around for about two years but only just getting popular! We have had one from since they came out and thrilled with it! My little on Sophia sleeps all through the night no problems. I'm told they are very good for babies with allergies (although sophia isn't allergic to anything so far) because they are chemical free and breathable.
Anyways, try the following:
www.thelittlegreensheep.co.uk
www.nct.org.uk (great for more info!)
Tim