What mattress should i go for

HELP!
I am a large lady of 23 stone, now obviously i suffer with severe back and hip pain. With my current mattress it has kept the support (good edges that have kept shape, no roll with only slight indentations after 2 years of use) but i do find that it is too firm for me. I wake up in the morning with bad pain that can render me stiff and immobile for the rest of the day. I also get very hot during the night. I believe this mattress is a Silentnight Miracoil 7 Hepworth Memory kingsize mattress.
There are many conflicting sites out there that say memory is the best way to go for the larger people but i just dont know. Its a minefield out there and close to giving up trying to find the right one. Do i go for a medium feel or a firm feel knowing that the one i got right now is very uncomfortable. memory foam, latex or pocketsprung arrrgggg.
Please help