//^^^ Emma's guarantee covers "cracks, dents or other damages to the foam that have occurred despite proper use and handling of the mattress".
So, if Scarlett thinks that she's covered, she can get a replacement mattress simply by sending a photo of the damage:
https://www.emma-mattress.co.uk/10-year-guarantee/
However that might not help much if the replacement mattress is also going to sag.//
A refund is also an option and it also covers:
19.2 Our 10-year guarantee covers your mattress against the following faults:
19.2.b Any permanent sags or dips that occur within your mattress despite correct usage. The indentation should be greater than 2.5cm and visible to the naked eye. This does not include standard softening of the mattress after extensive use and is only deemed faulty if a permanent indent is apparent.
Though of course you still need to prove all that!