After having many aches and pains using my previous mattress I decided to order this having read good reviews on it. So glad I did! It is so comfortable and has made a huge difference to my quality of sleep as well as reducing pains in my back. 
It does indeed expand to 20cms as stated and the quality is superb. Very happy with the delivery service and would recommend not only this product but also the website for buying. Thank you Wayfair!