I have read here sometimes about that viewing window for the air-con unit and how you're not supposed to see any bubbles, so I thought today I would have a look.

But I didn't know what I was supposed to see! ( this reflects my technical nous btw)

While the engine is off, the chamber is empty. When the engine and air-con are on, it briefly filled with what appeared to be a milky coloured fluid, then emptied again.

Is this normal? or is the air-con kaputt?
Can any of you technical gurus help me out?

Cheers,

it should look clear..if bubbles appear then it needs servicing, there shouldnt be any change in its colour

Yeah, as Matt says, should be clear.
Just had mine 'gassed-up' today by a mate who is in the air-conditioning game.
The viewing window is (on GS anyway) on the High pressure side of the circuit, indicated by an 'H' on the valve. This is where the window is, and is situated near the radiator. The top-up valve is on the Low pressure side, indicated by an 'L'.
My fluid in mine was green, which indicated that a substance had been added to find a leak, and therefore the fact that I needed it gassing-up (if it wasn't already obvious) was because of that. He did say that as it had taken 6 months to require a re-charge, it was probably easier/cheaper to just top it up once a year.

Could see bubbles as the air-con was circulating, during the gassing-up, which eventually went clear/steady when done. Only took 5 minutes, and a tenner for a few beers.