What's the best replacement mattress for us?

We have a memory foam mattress bought a couple of years ago which is dipping in the middle. We hardly get any sleep right now and wake up with terrible back pains every day. Every night I feel that I'm sleeping on a slide and believe that we need to replace the mattress urgently. Our bed base has slats which are 2 inches apart. My husband is 14st and I'm 9st. He's 6 ft tall and my self - 5' 3". Since we started using our current mattress I've felt very hot while sleeping and he's been alright . Please advise on which mattress we should go for. Our budget is around £800. Thank you.