Which Mattress

MouldyOldDough

We're keeping the original pocket sprung and adding Silentnight Impress Memory Foam Mattress Topper - https://amzn.eu/d/2p1blAJ
Should be reasonably comfortable 

NeverTooLate

If this is for a spare room where you will have different people visiting, I would not use a foam topper or mattress - in my experience about half the world hate foam (me included - too hot). So if you want to please everyone don't bother with foam.