Well, I phoned them today and now I know how to use it!
The leaflet is pretty bad - I was totally misled by the picture of how to fold the booster: half folded for a boy, double for a newborn, unfolded for a girl. Eh??
Currently I'm still using just the slimline booster pad at night, and he's been dry every night for 2 weeks now. Not sure what we're doing differently.
So, a question for the Nappy Guru (sounds better than geek? ). How do the pop-ins compare on price to other nappies?