I've just read that to qualify as a failure, tests want between 25mm and 40mm permanent dip.
For anyone that is curious, my Hyde and Sleep foam mattress had a dip of 3.5mm after 4 months use.
The tests are defined by the National Bed Federation (lol, what?!) and all shop's uses those defined tests as standard for situations like this.
For any kind of foam mattress, they wanted at least 2.5mm of permanent compression before they would call it a fault/warranty issue, so I was well over tolerance.
He put a string over the worst part and measured the depth, took lots of photos (any middle-width vertical bed support, slat width and the impacted area on the mattress) and did a 'lie down' test to get his own judgement. Wrote a report on a phone app and submitted there and then, start to finish was about 15mins.
dude said that the hybrid ones (1/2 spring and 1/2 foam) are the worst/hardest. His test wanted something like 3.5/4.5mm to qualify which are also apparently harder to test; and advised against them for that reason.
So next, Dreams should contact me within 5 days and book a replacement. I dont mind trying a foam again but it wont be from the same manufacture. I did think id have a fight on my hands but so far, so good. Pro-tip. If you have a foam mattress, take off the cotton cover so you see the actual foam; it's better for the photos.