What is the thinnest/most shallow pocket sprung mattress?

We need a zip and link pocket sprung mattress 5ft X 6ft .6ins to fit on our sprung King size divan base .
I have bought a thick memory foam topper so the mattress needs to be quite thin or we won't be able to climb up into bed!
(The problem is that I ditched our last mattress in favour of an open sprung mattress and it's like a trampoline!)
We are both in our 80's full of aches and pains and have had a stairlift fitted so not much room to get upstairs with a full size mattress?.......