What mattress do you recommend

HiWe need to change our mattress and after reading through all points on your website we think that the Artisan Natural would be the best choice.Here are some facts, Our existing mattress is a king size Hypnos bought in 1998 !! To overcome the dips on either side we fitted a Dormer Topper but it hasn't helped much. We are both in our late 70's and weigh 12 stone and 9 stone. The two piece Divan base is sprung and is divided across the middle (Not lengthwise). Do you think we should continue using the topper recently purchased on the new mattressYour comments would be greatly appreciated.Finally, do you dispose of the old mattress