Hypnos Mattress for Back Sleeper with no compressed upper layer

Dear TMU,
I need some expert advice please after a mattress purchase gone wrong! I wish I had found you before purchasing!!
I am a back sleeper. I used to have a Harrison Spinks Middleton Mohair firm mattress and after a few months in, and regular turning, once my body started to mold into it, I would get back pain. This led me to believe that as a back sleeper, I do not like a compressed upper layer or I like my mattress to hold its shape. After 5 years, which was too soon to change mattress, I had to change as I was rotating and turning the mattress every 3 -4 days! Madness! But this was the only way to not have any dips in the mattress surface and relieve my back pain.
I went to a local showroom in Cambridge (UK) and I told the salesperson exactly what I said here, that I need a mattress that will not mold to my body as that is what causes back pain and I was directed to the extra firm Hypnos Orthos Support 7. Extra firm made sense to me as I hoped that would hold its shape in the upper layer more than the firm mattress I had. I lay on it for about an hour and it was very comfortable and so I purchased it. I had to wait 8 weeks for delivery and at the point of delivery they took away my Harrison Spinks to be recycled.
On the first night of sleep, I had a terrible night of discomfort! I work up in the morning to find that my body had molded into the mattress after 1 night and I had really bad back pain!
I can return after 30 days if not suitable, luckily. I am now nearly 2 weeks in. I have changed pillow (with the firm mattress I used a panda memory foam pillow which works perfectly when there are no dips in the mattress but is not working with this new molded mattress) and I am getting some relief but I am sleeping on my side and back and waking up constantly in the night and in the morning I have shoulder pain. This mattress needs to go. It is not for me.
I know Brand shopping is not good but now I am stuck with this retailer who only sells Relyon and Hypnos. I can exchange my mattress for another mattress and I need to get it right this time. I don’t trust the salesperson to know as much as you people do as I was obviously sold a dud that was not in line with my PPP’s as you call them! I reached out to Hypnos last week too and I have not heard back.
The top layer of the Orthos Support 7 is made of wool and it appears that this layer compresses even though the firmness is extra firm. I assumed extra firm to mean that the top layer holds its shape. But what do I know?! Mattress purchasing is a mind field and I wish I had come across your forum before I purchased!
I know you can’t recommend particular mattresses, but I think you may have had this issue yourself and wondered if you know of a mattress with these brands where the upper layer does not compress around your body and holds its shape and is firm too, to match my PPP’s?
Any help would be very much appreciated.
Yours,
A distressed, back pain suffering mattress purchaser!