single mattress that feels soft

My name is James and I a 5ft 11 and weight 12.5 stones. I am looking for a replacement pocket spring mattress. The one I have is an 'Iron Bed Company' mattress sold as a 'medium firm' mattress. I sleep on my side and am finding my hip hurts after a while where it is in contact with the mattress and wonder if I need either a thick mattress topper or a new mattress that has a deeper comfort layer on top. Could you recommend one of your mattresses please? Thank you.