After a few hours of a "hissing" aircon system it no longer makes any noise, bonus. Although, no noticeably cold air, but it's not exactly hot outside right now. 

Problem, I'm not sure when the compressor clutch should engage, would it engage all the time? Is there an ambient outside temp when it won't engage, so as not to be needlessly using the aircon?

I can see the pulley turning, but the clutch doesn't. 

Is this likely to be a low pressure lock out? Can I bypass the relay/fuse to see if it does engage before I spend money out on a regas?

Cheers!

 

It doesn't engage below 4c, but it's not that cold here at least.  

You can force it on by bridging out the low pressure switch.  Literally just unplug it and put a jumper wire or an unbent paperclip between the correct pins in the plug then start the engine and press the AC button, the compressor should instantly kick in and the engine fan should run at full speed.

Most likely just low on gas so if the compressor does kick in by bridging out the switch, don't leave it running for long as it could cause damage.

There will be a thermostatic switch in the system which cuts it out when the evaporator temperature is just above freezing, to prevent it icing up. In normal operation this means the compressor will clutch on and off periodically.

You also have a couple of pressure switches to inhibit the compressor if the pressure is too high or low. Get the gas level checked as a first test.