We currently have a mattress 3 years old which has nested.

Hi Nancy,

Settlement is a natural occurrence with mattresses which is why they need to be turned and rotated frequently when new. Dips and indents, where the mattress has dropped below 2cm is a different matter. We have an article on mattress settlement here which goes into much more detail.

If you feel your marks and spencer mattress is faulty its best to contact them directly to discuss.  We recommend all our mattresses are turned and rotated monthly for the first 6 months at least. If you can continue to turn after this it will extend the life of your mattress.

Hope that helps.