Silentnight mattress with hump?

We have a slatted wooden base (Super King) which I made with 3" gaps and a central supporting bar. Currently we have a 5 year old(ish) Silentnight Miracoil sperung mattress - Rosemay model. Lately I have been having v uncomfortable sleep and back ache. Noted your comments in earlier posts about thin mdf sheets and topper. However, we have a hump in the middle of the mattress. Also where we sleep the mattress appears to have lost some of its support ie sunk. Have thought about a topper but not sure if this is the ideal answer?

Or, do we have to bite the bullet and replace the mattress and if so with what?

Any help would be appreciated