I found this gas fitting (pic link below) while I was ripping out some
old pipework which seems to have been sealed with something that sets
hard and is permanent but without the thread being driven fully home,
so you can get the fitting at the right angle.

http://i18.photobucket.com/albums/b104/mkpb/fitting.jpg

Looking at Toolstation's website, I've found Instant Gasket,
Threadlock and Liquid PTFE.

http://www.toolstation.com/shop/Adhe.../sd2340/p61999

http://www.toolstation.com/shop/Adhe.../sd2340/p34358

http://www.toolstation.com/shop/Plum.../sd2711/p36639

Can anyone tell me which one has likely been used here for a permanent
solid joint (and what the other two would be used for)?

Thanks in advance.