Would a wool topper improve our mattress nightmare

Hi guys, loving the website it's a very useful resource! We have a brand new John Lewis Rouen bedframe with sprung slatted base. We are healthy early 30s and both 10-11st. I'm 6ft. We vary from side sleepers to back during the night. We purchased a king Duvulay Hilary Devey Diamond mattress after "testing in the shop" and a glowing online newspaper review. This arrived with some faulty stitching and was therefore due to be exchanged. We felt it was nicely supportive but not quite soft enough to prevent slightly sore pressure points on bony hips. Nitpicking i guess after 9 years on a broken mattress. Rather than buying a topper we instead exchanged it for the Diamond Luxe mattress after speaking to their customer services. This has been an unmitigated disaster. It's rated as "medium-feel" but it feels far less supportive than we expected, completely different to the diamond. It's noticeably lighter and bends more easily. It sinks into a small depression where you lie on it, but it seems to be more the springs compressing very easily than us sinking into the upper layers as we were expecting. The springs are quite suprisingly audible when compressing the top with a hand. After a week we are both waking every morning with upper or lower back pain depending on sleeping position, which we've never had before in our lives.
Duvulay and the retailer are not willing to help. They won't even allow us to transfer the warranty if we try to sell it on at a few days old. They suggested an expensive memory foam topper might help but we can't stand memory foam and like a "traditional mattress" feel. Do you think boarding over the slats with MDT will help (there is a duvet protecting the underside at the mo) even though the distance between is less than Duvulay specify? Would expanding the comfort layer with a 2 inch woolen mattress topper add a bit more lumbar support if we were have a bit more to sink in to?
Thoroughly regretting our choice of brand, wish we'd got the Vispring Realm we originally intended to!