I've come to the conclusion that PET bottles are crap for long term conditioning.
I made an oatmeal stout at the end of November but for some reason I cant work out it had an astringency to it (there seems to be a number of reasons for astringency and I couldnt pin it down),- fortunately I only made 5L. So I just tucked them away in my bottle storage area to hopefully condition out the astringency. This actually worked for the most part, however all the beer in the coopers PET bottles I used had oxidised.
Now there can only be two reasons for this i) I didn't screw the caps down tight enough - but the carbonation seemed fine for the amount of priming solution I added ii) Cooper Oxy-bar PET bottles aren't as oxygen barring as you'd want.
I also filled 3 swing top bottles too. I've just opened one of them. Definately not the best beer in the world, although the astringency has decreased significantly but no oxidisation.
Fortunately I'm in the process of replacing all 200 of my coopers PET bottles for glass