Leaking Crox Nut.
This leak had been going for a while.
This connection was leaking badly. After attempting to seal the join with hemp and graphite it was still leaking. I had to remove the copper pipe connecting the hot water cylinder to the tempering valve and take it into the light. The brass had split down the seam. After cutting the pipe down slightly to apply new crox nuts at each end, I married the joins back up and voila!
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