Knew I'd read a thread about this in the past, excuse me bumping it!
I've been using older Avent bottles with no leakage for months now. Bought a new (BPA free) bottle last week, and it leaks like a flippin' river!
Done some online research plus some testing, and have found the following.
When the BPA-free bottles were first launched, they suffered awfully from leaking. There is a method of lining up vents & flanges & tightening to a certain point, but try doing that in a hurry with a screaming baby.
The newer BPA-free bottles come with an extra ring, supposedly now a "two-part anti-colic system". And yes, using this ring stops the leakage. I'd left it out when I was using the bottle as my babba has no problems with colic. Now I'll have to ensure I use it.
Looking carefully, all that ring actually does is increase the height of the bottle neck above the threads! And then comparing it to my older bottles, I see that the new BPA-free bottles don't have a deep enough neck to press onto the teat and hold it in place within the ring.
"Two-part anti-colic" system, my a*se. Avent realised they had designed a duff bottle, and had to design an extra part to remedy the problem.
[vent over!]