Mattress advice for side sleeper

Hi. We are looking to replace our double (4ft6) mattress. The one we have is very firm, we have tried to soften this with a memory foam topper, but this seems to cause excessive heat, and I still wake up with lower back pain. I am a side sleeper and I am convinced the hardness of the mattress is the cause. I'm not unusal in weight or height (5' 11" and around 13.5 stone, my wife albeit somewhat smaller!). I was in the department store Next recently and tried their "2000 Pocket Sprung Ultimate Natural Pillowtop Medium Mattress". This was advertised as "2 - medium", with a wool/cashmere/silk pillow top. http://www.next.co.uk/x571294s3#667282?i=double.
I have to admit, when I lay on this bed, it was just the right softness. I felt my back was supported well as a side sleeper, it felt amazing, convinced that a natural fill pillow top was the way to go - I almost pulled the credit card there and then! I didn't though - I came home and checked on line.
It would appear that a good many people have suggested that pillow top beds (Next included) can suffer a lot with sagging, and that a better solution would be a good mattress and a good topper.
At this point I stumbled on your very informative web site - you guys seem to really know your beds! Can you suggest to me a bed and topper that you offer that would give me the same experience I felt when I lay on that bed in Next today?
Best regards