Thank you for your help,

I went to Hopespare hydraulics and pneumatics yesterday, and the screw in elbow is in the process of being replaced with a compression fitting and a flexible, thermoplastic hose. The fire extinguisher valve small thread was also identified as a 7/8 unf 14tpi if anyone is struggling with this! (The larger thread into the bottle is an M25 x 2, though i couldnt find anything commercially available to fit this.)

The hose will then attach to a 3 way T ball valve, to isolate or dump the bottle, which can be clamped onto the chassis.

He has also supplied me with a quick release that does not seal when released, so no pressure will be stored.