We bought a king size marks and spencer divan and mattress for our spare room which is really comfortable, so we bought the same for our bedroom, but the double version as we don't have space for a king size. Straight away, it was much less firm and not as comfy as the king size one. Over time, despite regularly turning the mattress it's become saggy, lumpy and dips in the middle. It's very bouncy. I hate it. This week I've got really bad back pain which I think is partly down to our bed.
We really need an upgrade. I can't decide whether we need to replace just the mattress or both the bed and mattress...I don't know how to tell.
We don't have room for a king size 
I have no idea what to go for, there's so much choice and we don't have a huge budget. We could probably stretch to £2k if we had to, but would rather spend less than half of that.
Help please!