Finally got the answer I needed from a friendly welder. You need to unscrew the input stem and replace the US nut with a UK one.
Sounds simple. However, the stem is VERY tight; I needed to put a pipe on the allen key to shift it.
That done it would appear to be a simple job.
Then all I needed to do was track down a nut, again easier said then done.
In the end I managed to get a nut off a CO2 fire extinguisher. It's the nut that holds the horn on.
Only problem was the pipe on the horn is smaller on the CO2 fire extinguisher than my stem.
I recommend getting one off an old CO2 fire extinguisher as they are typically brass and slightly bigger.
I then needed to drill out the center from 15mm to 16mm to fit my stem.
And then finally put it on my cylinder, the final piece of the jigsaw........
I now think the nut in this link would work although they were not helpful when I asked for details on the nut. www.weldability.com part no
DACO2NUT
I'm now slowly bringing the ph down in my tank.
I am a happy fish keeper once again.