Choosing a mattress for someone who is bedbound.

Hi there,
I hope you can help. I'm looking to replace our current latex topped one sided mattress (I know!) as it's developed a "sag" in the middle of my side and is now about 9 years old. What complicates our choice somewhat is that I actually spend pretty much 24 hours a day in bed as I have a long term illness. I also have, just for a little extra sprinkles on top, arthritis in my hips and a chronic pain condition called fibromyalgia. I tend to sleep on my side and often wake with pain in my hips which I think is exacerbated by the mattress as I've slept in beds in other places which don't hurt me (the Premier Inn Hypnos mattresses are very comfortable although I don't particularly want another pillow top - I just mention this in case it helps with calibrating my comfort level).
With our current mattress we use a 6cm dual layer feather mattress topper which has certainly helped with comfort levels, but I think I would struggle with moving around on a soft mattress - muscle fatigue means changing position can be difficult. I also find that I am affected by my husband moving around on his side of the bed - I get jiggled and bounced around which I find very uncomfortable, despite how careful he is in trying not to! I looked into getting zip linked king size mattresses, but unfortunately these are outside our price range. We have a king size bed frame which we are keen to keep, and we have a restricted staircase which means we cannot get a divan bed up to our room, so we will have to stick with a king size mattress.
We are both of similar height and weight - 1 am 5'10" and he is 5'11" and we both weigh between 15 and 16 stone. So, is it possible to recommend a comfortable mattress that will be able to stand pretty much 24 hour occupation and that can reduce movement between partners without compromising on support? We are presently considering either an Origin or an Artisan 1500 but any advice on choosing would be great.
Thanks so much in advance for your advice :)