i find the most effective way is to pour boiling or very hot water on the neck of the bottle, which softens the loctite. Then with the help of another, unscrew the reg from the bottle.
EDIT : Boiling water didn't do it so I used the rubber end of a hammer and tw@ted the fill nipple until the reg came loose. I figured it's cheaper for a new fill nipple then a new bottle and reg . And guess what, the fill nipple is still fine. BTW - if anyone else does this - don't blame me if you fcuk it up!