Silentnight Deep Sleep Mattress Topper Double Bed, Hollowfibre

This is the topper that proves the point of the whole ranking. It has the second-largest sample of ratings collected here — 18,386 of them at 4.4 stars — and it costs £21.34. It also weighs 1.2kg, which is less filling than the 5cm topper above it and roughly a seventh of what the memory foam toppers at the top carry.

To be fair to Silentnight, the listing never claims a depth: the title and bullets say Deep Sleep, enhanced comfort layer and soft support, and nothing in centimetres. The trap is in the search results, where a £21 topper with eighteen thousand ratings and a confident name looks like the same product as a 10cm topper. It is not, and its own weight column is how you can tell.