Just buy one!
Having bought a mattress from M&S, twice as expensive as Leesa around 3 years ago,that developed two bad indentations in the memory foam top, my husband I have been suffering from aching backs and necks, and badly disturbed sleep. I saw the Which? recommendation and the excellent reviews on the Leesa site and, with the incentive of money off we went ahead and bought one. Well, what can I say - no more aches and pains. No more insomnia. This is a terrific product. We found the chemical smell quite strong to start with, and it took about 3 weeks to go but other than that it has been brilliant from the unboxing. We can't recommend more highly. Go ahead - get one!