Now I'm unsure. Should I return this one (still boxed) and get some as my other or ignore. The mattress it comes with isn't what I would call a mattress.

Red Kite Travel cots | Argos

Because the foam was slightly bigger than the cot, it was a very tight fit around the base. That meant that my son couldn't in any way get trapped down the sides, which I think is the main reason they tell you not to use another mattress.

Little one been using travel cot from 4 mo as put grew swinging crib. Have the sleep n play. Bought a mattress and use newborn insert. Many years ago I bought a Graco travel cot which also stated that you should only use the mattress that comes with the travel cot. That mattress was about as comfortable as a plank of wood. So I measured the internal size of the base of the travel cot and added on a centimetre or 2 each way. I went to a local foam furnishing shop, explained I wanted the foam for a travel cot, they cut the foam to the size I wanted and used a type of foam that was suitable for a new born to sleep on.