New bed/mattress advice

Hi,
My wife and I are looking for a new bed and mattress. I'm 6'6" and weigh 19 stone, my wife is 5'7" and weighs 12 stone. We're assuming a zip & link with one half firm, one half medium would be appropriate (after reading your comprehensive website!) We'd like to go for a super king size with some storage..
I also have problems with my back and hips, and sometimes find it quite painful to lie on my side due to the pressure on my hips, so I think a softer feeling top of some description would be good.
Our current bed is a king size with a single piece memory foam mattress - not sure of tension, medium maybe, but it hasn't stayed comfortable for very long.
I'm really glad I discovered your website, and would be really happy to take any advice that you could offer, including suitable options from your range. (We've just been to Dreams and left with a view of never returning...) Getting the right mattress is our main concern, budget comes a very close second to the most suitable option.
Many thanks,
Marc