Something to soften my bed

I am a 10 stone (54 year old !) man who likes sleeping on a soft bed.
I have moved into a house as a tenant and find the mattress on my bed too hard.
I've tried sleeping on a sleeping bag which isn't bad but I can feel the hard bed mattress underneath.
Can you suggest a solution for me - say something to put on top of my bed to make it soft so I can't feel the hard mattress underneath at all?
Thanks.