Salopian,
What you said about many sources of cch in the UK made me think of our sources here in the US. I think there are more sources today in the US than 20 yrs ago when I sent my first double frame out for cch. Back then, there were only a couple that I could locate. Today, there are many that I can cite just off the top of my head. Most are doublegun smiths that have learned the process.