Nice one UK, looking really green and healthy there, and looks like a good set-up you've got going. Glad your sunburn is sorting itself out. Couldn't you just re-pot the whole thing into bigger (deeper) pot, lightly fill in around the bottle with soil, remove the bottle by cutting down one side and carefully pulling it away and finally fill in with more soil to firm it up? If you give the whole lot a good soak with water (both the soil in the pot and especially the top part in the botte), then leave it for a couple of hours before you start, the soil should stay together for you when you take the bottle away. Even if it didn't so long as you've at least lightly filled in around it before you cut, the soil mass isn't really going to go anywhere. Just a thought, please tell me if I'm talking crap LOL.
I wish you all the best for the grow - you seem to be doing an excellent job. Keep up the good work and you're in for a treat

Look forward to seeing your next pics.
greenb.