Ok, the battery that I had was indeed a lemon, but it turns out that there is a problem in my gun too. Even with a new battery that the shop supplied (which was tested on site with a gun that is extremely similar), the gun did not fire.
Can anyone tell me what is the most common cause for a click but no firing?
Ares M110 LEAF - Ares Striker Tan
TM G17 Pink - KJW P226 TwoTone