Will a topper soften the feel

Hi,
I currently have a Hypnos pocket sprung ortho matteress, double on a pocket sprung divan base. I have found this too soft and have placed quite thick ply over the base making it solid.
I do still have slight back pain from the bed and my partner complains of movement when I turn over. I am considering a 100% latex topper to help the general comfort of my bed, I  am concerned that a topper will soften the bed further.
I am 5.10 and 13.5 st and my partner is 5.4 and 10.5 st. What topper (if any) thicknesses and firmness would you recommend, thanks.