OK, there's no other way to say this so I'll just say it. I'm an eejit. The bottle wasn't empty, I just wasn't turning the valve hard enough. It twists a bit quite easily then gets very stiff, at which point you have to twist it hard to open it up. This was demonstrated by the LFS I bought it from this morning after I went in complaining about "you lot selling me empty co2 bottles". Naturally you can imagine what an blahblahblahblah I looked. Apologies to those who have wasted their time on this thread and, of course, to LFS.
Much worse is that I have had to have a blackout in the tank yesterday. Didn't want lights on without co2. Came back early afternoon to find fish and shrimp at water surface and three dead red nosed shrimp on substrate. O2 shortage. Why? Well what's the major source of O2 in my tank? The plants. And what happens to photosynthesis when you turn the lights off?
Ran an airstone under Koralia and flooded the tank with air bubbles. Looks like I avoided any further casualties. Careful with blackouts when not taking precautions. CO2 now setup, hopefully I should have mildly fizzy tank when I get home tomorrow (new up atomiser but very low bubble count to start with). I'm just going to say it again blackboard punishment stylee to make it absolutely clear:
I am a fool
I am a fool
I am a fool