Does anyone know how to measure a cot for a mattress? Mama's and papas cots come in 3 sizes, 200, 300, 400 which means absolutely nothing in real terms. I'm looking at buying a mattress from Argos as I have vouchers and there cheaper. The only thing is they measure them in cm not the magical m&p sizes. I know there's something about size of gaps around the edges but can't remember what they are supposed to be. Can anyone shed some light before I give up??