You are so right; the advice for people with back issues used to be to go for a firm mattress, however, recent advice from the Institute of Chiropractors contradicts that.
Ideally, you need support for your body weight from the spring unit with the upholstery providing the comfort and in your case that “sink in” feeling.
Although you have suggested you would prefer a soft mattress, I would recommend our Origins Pocket 1500, which has a medium feel in the spring with softer comfort layers of upholstery. This currently retails at £480.00 for a double and I would suggest you view the details of this product on our site prior to placing an order.
This is ideal for your body weight, but in the event that you find this slightly firmer than required, you could then apply a topper.
I hope this has answered your questions fully but if you do have any further queries please don’t hesitate to contact our office on 0161 437 4419.
Kind Regards, Mike.