I'm sure this comes up a good few times. ST150 08 plate.

I know mine has gas in it, and the light comes on when I turn on the AC, so the fuse is fine. However, nothing happens, no action under the bonnet, radiator fan doesn't come on, nothing really happens.

Is there a relay I should be looking at, or anything like that, electrical or simple, before giving up and taking it to the garage?

There's a noticable clunk when the AC kicks in, which I assume is a relay. That's about all I know really, sorry. You could double check the connector is fitted solidly

next time mine is for air con I will ask them to top it above the limit on the label and we know the unit takes a lot more, see the top tips on this, probably why mine is never really cold and I no longer here the ac kick in properly, I remember when it was newer and I pressed the re-circ there was a noticable boost.