That screw is actually the emergency bleed. Its not good to keep using it though, its only meant for when the batteries die. It'd probably be better to use the electrics.

That's my last resort, to buy a different brand valve, but even that's proving difficult in my area. I just wondered if anyone in the UK managed to get hold of a valve.
EDIT: Just found out they ship worldwide, thanks for all the help

Tried that, no dice

I'm sure peeps over here in the UK have made a pneumatic cannon before, but I'm having trouble getting hold of a rainbird sprinkler valve.
Does anyone know where I can get one, and which is the best one to get?