Ikea Morgedal firm mattress too firm!

Hi,
I recently purchased a double wall bed and an Ikea Morgedal firm mattress (a plain foam mattress) to go with it. I purchased the mattress based on the good 'Which?' mattress review and also because it seemed comfy in the shop. I have now sleep on it for a week and am struggling because the mattress feels too firm. I feel like I am lying on top of it rather than sinking in - it's a bit like sleeping on the floor, to be honest. Anyway, I was hoping you might be able to suggest a solution to help me get a better nights sleep? I was thinking of either a replacement mattress or perhaps a topper.
My old mattress was a pocket sprung one with a latex top layer (attached - not a separate topper). I quite liked it as it felt like I sunk into it nicely. I don't know whether it was really appropriate for me, however, it was comfy and I got a good nights sleep without any backache. I would describe it as a medium.
I am 6ft 2 and weigh approx. 15 stone. I sleep on my stomach all the time. My new wall bed will take a mattress up to about 25cm deep. My Ikea Morgedal mattress is about 18cm deep.
I have looked at your fusion range of mattresses but don't know whether they would be appropriate for me, or which of the range would be best. I was also considering your latex mattress topper to go on to of my existing Ikea Morgedal mattress, however again do not know whether this is appropriate.
Any help of pointers you could give me would be much appreciated.
Thank you.