Ceb - that is odd! I am sure there is a simple answer. DD2 is smaller than that, under 8lb and we haven't had a single leak yet and have similar nappies. We have had no problems with BG or Sandys or TB. AND we have got on fine with tots wraps! Although we are now using Nature babies wraps but only because they are so much prettier! Lol.
So, what have you been doing? The BG, do you fold them down with the poppers for the smallest rise? And you are using the newborn inserts inside? Where are they leaking? If it is the legs, have you tried fiddling about getting a good fit when you put them on? We have to do it a couple of times to get it right, but then don't have leaks.
With the Sandys - what are you washing them in? And how much washing powder etc? I have found them pretty bombproof to be honest and have just bought more. We have used them with the tots bots wraps with no problems. We get a 3-4 hour stretch if she doesn't poo, and have never leaked, not even just a bit on the wrap or anything. Are they the Extra small ones?
Haven't used the pop ins on a newborn so can't help there. I have lollipop nappies to try but haven't gone down that route yet.