The only problem was, I couldn't get it to hold gas, despite cleaning, vaselining and tightening up all the seals till I was blue in the face. Soapy water failed to show any leaks around the valve joints so I was blaming the o-ring, but despite putting a new one on, no joy.
Eventually, I accidentally brushed soapy water against the side of the carbonator cap, and guess what? It had a minute crack in the body which was letting gas pee out.

I was stumped for a fix (don't want to buy another carbonator cap) but eventually came up with this solution using a JG to cornie disconnect fitting I had lying about. I was dead lucky to find a bolt that fitted it in my odds and sods box, and together with a couple of washers and seals I also had lying about, I think I've sorted it (at least it seems to be all holding pressure now).
The only problem is that to change over from gassed-up to 'on the cask breather' I have to switch off the gas, take the pipes out of the JG fitting and blank off the one not in use. Once I buy a couple of taps should be OK though.