mike wrote:
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.

The angle fitting looks like a standard 1/4" taper B.S.P. (other bit
is a 'nipple') and sealant is optional to the mechanical assembly
aspects. The red stuff ISTR as "Stag". Used buckets of the stuff on
gas and instrumentation piping. Needed dry, clean surfaces, was
horrible to paint on, it stunk and dissembly was a pain-in-the-bum.
PTFE thread tape when it turned up on the market was a true
revalation. Stag was dumped within weeks.
The fillers you show all look like very pricey cyano type adhesives.
I'd be inclined just to use cheap PTFE thread tape