In reply to DeLorean:
i checked what the big brands were using and bought some of that. Warp-knitted spacer fabric it’s called apparently. I found a distributor that sold 30mm depth off the roll so bought enough for two and cut in half. I bought some other non-warp-knitted slimmer spacer fabric too. I put the combinations of 30mm, 5mm, 10mm and another 5mm into a duvet cover and it works a treat. For the kids one, I just put 30mm and 5mm and it was fine. Sadly I can’t find any detail of the product name or the distributor, sorry.
Edit: sorry, the reason I wanted cot-safe breathable was that my youngest at the time was borderline cot age. Otherwise I’m sure a length of memory foam would have done fine. I’m glad I used the warp knitted stuff a few years down the line, though, it’s held consistency and bounce nicely.
Post edited at 13:23