I think the born free ones are very similar to Dr Browns ones. The leaks happen when the formula is warm, and the air pressure causes the milk to come out through the venting tube.
I found that unscrewing the lid slightly when it's hot or being warmed stops the milk going up the tube. So apart from when you're actually feeding your LO, always have the lid slightly loose when it's warm.
If you need to shake to mix, have it ever so slightly loose then turn the whole bottle upside down and tighten at the same time. Shake upside down, then turn it back the right way when you've finished. It takes a bit of practise, but I preferred that to stirring it, which is the other option.
