We bought a very expensive mattress which after a few months both sides of our mattress dipped significantly. My side was worse than my wife's despite the fact two nights a week I'm not there as I work away. We contacted the store and the sent two people round to have a look at it. One of whom was a manager of the store manager. He agreed it ha dipped significantly, over 50mm. He arranged for an "independent" bed specialist to check it. They decided it was an acceptable amount of settlement. After months of bad backs and difficulty turning over in bed we got rid of our £1000 mattress.