What mattress would you recommend for 10 st and 17 st sleepers

We have a king size slatted base bed and would like a new mattress. I am 17 stone and my wife is 10 stone.
Our budget is around £600-800 tips.
We currently have a 10 year old silent night New York pocket sprung with a separate memory foam topper. I hate the memory foam as it makes me sweat and the mattress is now totally knackered!
Over to you please, as we're now addled by the choice and extortionate cost of Hypnos or other big brand beds - I like you open and honest site and would like a good night's sleep !!!! :-)