Do not honor their warranty terms. Review on Sundayrest

I bought a mattress 20 months back(March 2017), The mattress was lovely for the initial few months and sleep was brilliant. However in 16 months or so, the surface became uneven and there is now a depression on the sleep area, making the mattress hard in the centre and soft on the sides(this is a queen size mattress). Rotating helped for a couple of months but eventually it was of no use. Sunday agreed to send a technical team for inspection after we sent them pictures. They came with a hardness testing device(whatever that is!) and tested in 32 positions with hardness varying from 39 to 51. They came up with a table that averaged the hardness by taking the median as 100% and the hardness varying from 88.8% to 112.2% and declared that nothing is wrong with the mattress. No idea what the validity of this method is, what the hardness is supposed to be, what happened to their 1 inch warranty clause.(The depression is definitely more than an inch). Going by their method(even if it is hogwash), there is a difference of 25% in softness in different areas of the mattress, not sure how that is considered normal! I now have a mattress that leaves me with back pain every morning.