New Super King Hypnos mattress is too soft causing back ache.

I have recently purchased a Hypnos pocket sprung, pillow top mattress and after 6 weeks use I'm finding it too soft causing back ache. The retailer will not exchange and so I'm looking at a way of firming it up. Is there a firm mattress topper I can buy which will resolve this? I am 16 stone and 6'2". Thanks in advance