what is Easi Pocket Reactive Foam, will it sag and who makes it?

What is Easi Pocket Reactive 1250 mattress made of, will it sag within one week and which UK company makes it - is it Reylon? (I'm having huge problems with Benson for Beds and so far, 2 mattresses have been completely unacceptable! But, unfortunately, I have a budget to adhere to). I would be most grateful for your answer please.