Jonno.. you got any local fire exstinguisher companies? Theres one by me (CFS) ... They gave me about 4 fire exstinguishers last time i went... the black nozzle thats on one as standard has a co2 nut in it with a 1/8th male bsp thread on it when you un screw it...

If not- ring Paul cooper up, technobots used to stock them but i carnt find them on there site now?