Well, our second trial of the WNNN wasn't too successful. I made sure there was no leg gape, etc... and he was soaking when he got up this morning. The nappy was wetter this time, but he was soaked up his back. Last time the body was dry-ish, indicating he'd simply peed straight out of the side as that was where the wet was. I also suspected it might have been where he'd had a two-piece pyjama set on last time, and that was pulling on the top of the nappy/wrap.
I'm beginning to think it's the wrap. I've only got a large WN wrap, and it's pretty small over the large WNNN. I believe it's compressing the nappy slightly, then with him lying on that on top, it's just too much. It's the only thing I can think of as I put him in a sleepsuit too last night. I'm waiting on an XL wrap, so think I will shelve the WNNN until it's here. I refuse to believe it's not absorbant enough, as we had no leaks at all with a bedbug, and that's verging on being way to small for him!