Bed not supportive for my increase in body weight

Hi
We recently bought a Somnus Jewel king sized medium or regular. It was delivered in November and was an absolute delight, but after a week or so I started to feel like I was falling out of bed. I called the dealer who I had purchased from and he suggested that I should be rotating weekly and flipping it over every two weeks. He also said that the mattreses get compressed during delivery and they take time to settle. He then said if it is still like it in 3 months then I should give him a ring.
Last year I suffered an attack of inflammation of the spinal cord and it has left me with impaired mobility and a number of other issues. The tablets I have to take and the inactivity caused by my being unable to undertake any exercise has made my weight shoot up to a weight of 19 stone. When we bought the bednit was very clear that I was large, but he did not mention anything about what we should go for in relation to my weight. My husband is 14 1/2 stone.
We followed the advise and turned and flipped regularly. By February of this year the problem was so bad that I fell out of bed twice. There was a huge hump in the middle of the bed and it slanted down on both sides. I contacted the shop and they contacted Somnus who then sent an assessor to look at the bed. It was stripped down so he could see the matteress clearly. As we walked upstairs I asked if they had weight limits on their beds and he said no, added that he would always advise larger people go for the firmer matteres. I told him my weight and he said that he was nearly 17 stone and he had a somnus matteress. As soon as he walked into the bedroom he said that this was clearly a manufacturing fault and that his recommendation was to replace it. We went to the store as soon as he was gone and told the sales chap what had been advised. We went back upstairs to try the bed in firm and decided to go for a firm Marquise matteress, the next grade up and costing an extra £245 making our total spend on the matteress, divan and headboard £2400 with the bulk of thus (nearly 1800 ) just for the matteress. It arrived on Wednesday and is very very comfortable. I am however frightened of it happening again. The answer I need is am I to big for my bed or should the 12000 springs be able to support my weight.
Thank you for any help
Kind Refsrds
Mickey Hunt