Ok, did a little experimenting last night and here's what happened.
Midnight to 4am DS wore a cotton prefold in bummi's cover and managed to get him asleep on his back = no leaks
4am to 8am (a little longer than he usually sleeps) BG OSFA with bamboo prefold instead of microfibre insert, 2nd time in a row he went to sleep on his back = wet patch wasn't just contained to hip area but spread far and wide like someone knew that I was doing washing this morning anyway! The bamboo was still wet though...

Gigi, we were actually given one of those double roll thingies from SIL but didn't quite know how to use it but will now be digging it out of the box from whence it came.
Also, am just waiting for my ecobubs to be delivered, they certainly look like a good nappy and people seem to like them but I wasn't sure if they would make a good night nappy given they had the layer of PUL in them (breathability?) Only one way to find out.

For the record, I'm a Melbourne girl and the city is living up to the whole 4 seasons in a day thing which is why dressing is confusing me at the moment. When I put layers on and take them off all day and night what chance does the little man have.