This is the method I finally arrived at —after a fair bit of trial and error— for safely removing the valves from EMPTY! Calor Gas butane bottles. Other methods I've seen posted online involve grinding the collar of the bottle off first, to gain access to fit a spanner onto the valve and allow you room to turn it. I didn't fancy wielding an angle grinder anywhere near a still "uncorked" gas bottle... even an empty one. So I decided to play it safer and customise a socket to do the job, instead.

I've also found a simple but effective method for wedging the bottle in place and stopping it from rotating, while you heroically heave on the valve to try and loosen it.

DISCLAIMER: Don't attempt this unless you're aware of the risks and know what you're doing. Even a nominally empty gas bottle has a fair amount of highly inflammable explosive gas left inside. I had previously vented these bottles several times on hot days, to allow as much residual Butane as possible to escape, before attempting to remove the valves.

TIMELAPSE MUZAK: Double You The Mini Vandals

AMAZON LINKIES to "stuff" used in video:

Draper 1/2" Drive Ratchet – https://amzn.to/4b3vLQO
Katsu Cordless Angle Grinder — https://amzn.to/3z0Eeq9 [not recommended]
Kielder Combi Drill – https://amzn.to/3Ucdrho
Roughneck Gorilla Bar — https://amzn.to/4eir73D

ALSO NEEDED:
30mm Impact Socket [I just bought a cheap one off eBay]
Cheater Bar or Scaffold Bar