Mattress suitable for a sprung slatted base

Good afternoon,We have just replaced our bedframe and have gone for a sprung wooden slatted ottoman. Both my husband and I suffer with lower back pain in bed and I think we could do with a softer mattress but would like some advice as I have read your section on slatted bases and am unsure what would be best. We are both side sleepers and not very heavy, I am 8 1/2 stone and my husband 10 stone. We are currently on a staples mattress we purchased around 5 years ago (which was to replace a Tempur mattress we found way too hard, plus we both got too hot on the tempur one).Can you please send me some recommendations, I as thinking probably a medium soft pocket sprung.Thank you,Laura