Hi Katy, 

Thanks for your question. 
 
Having looked at the specifications of your current mattress I can see that it's a pillowtop mattress. We would always advise to steer clear of these mattresses as once that fabric compresses, as it might very well have here, there is nothing you can do to salvage it. 
 
At your body weights you would be matched with a Medium spring tension to get the right support but the 59KG person is borderline between a Soft and Medium spring. If they want a bit more give then choose the Soft or for a bit more support choose the Medium. 
 

 
You haven't disclosed the size you are looking at but have a look at our Origins Pocket 1500 or our Artisan Naturals. Both have that true medium feel in the top layers giving that slight sink into the top layer but not too much. The main difference between them is that the Artisan range is constructed with natural fillings, which tend to last longer as well as providing a cool sleep surface. 
 

 
Front sleepers need to be avoiding anything too soft in the top layers as this can lead to an unnatural arch in your back making any existing pains even worse. 
 
I hope this helps but if you have any further queries please don't hesitate to contact our office on 0161 437 4419.
 
Kind Regards, Lee.