Eventually discovered I am an idiot! RTFM. Had 16 on high power (the default), and 67 on low power (also the default) when in the dual watch mode.
When I put the 67 as the only channel (off dual watch) I originally manually over rode the default to high power, since I had noticed it was in lo power. This is remembered by the set, so whenever you go into only chanel 67, it selects high power. So it worked fine.
Back into dual watch, and 67 drops back to the default dual watch low power on 67 (which is hard to see as it goes lo / hi on the indicator rapidly as switches it 16 / 67) unless you set it to high power when in dual watch mode. Then it remembers and stays in high power the next time you move to dual watch.
It does say so in the manual......