Millbrook Royal 3000 Mattress is too firm, any suggestions?

I have been reading your web site regarding the support level of mattress and mattress toppers and would appreciate some guidance. I have a king size Millbrook Royal 3000 mattress which is fine for my husband but which I find too firm. At least I thinks that's the problem; this is the second new mattress we have bought in a short period for the same reason (the first was memory foam and felt harder than the current one). I am 5'6 tall, weigh 8 stone and am an hourglass shape. When I lie on my back I feel as if there is a slight gap between my lower back and the mattress; on my side the gap is under my waist making my spine feel as if it is bending sideways. I'm thinking therefore that a softer feel would be helpful. My husband is 6' and weighs 15 stone but can happily sleep on anything (he has found both new mattresses very comfortable). I have ruled out buying yet another mattress but would really like to be able to counteract the firmness with a suitable mattress topper to give a bit of added 'sink in' to the surface. Please advise. Thanks, Clare >