Mattress for a lightweight back sleeper with a heavier partner?

I weigh a little under 8 stone and am most comfortable sleeping on my back while my husband is 4 stone heavier. My main problem is that my husband's weight depresses the mattress we have (even though it's a pocket sprung one) creating a dip which leaves me then lying on a slope - tricky on your back ... I spend the night on my side instead, trying to claw my way back up the slope, which isn't restful. Many years ago I enjoyed sleeping on a thick good quality futon mattress. I liked the fact that it didn't move about and that the top layer cushioned my bones but the underlying feel was solid and supportive for my back. I suppose I was lucky that my light weight didn't compress the filling too much. With our current pocket sprung mattress the firm surface sometimes feels a bit too firm for my legs in particular, leaving my calves feeling a bit squashed when I try to lie on my back. My current budget is around £700 although I could save up more if it would be worth it for a good night's sleep. Any advice would be appreciated. Thanks very much.