So I tested both thermostats and they both worked. After some thought I decided to put the Triumph OEM item back in. The Stant 13908 - when fully open - extends too far into the interior casting of the cyl for my comfort level. By contrast the OEM item doesn't extend at all. The Stant is 41mm from flange to base at full open but there's a shoulder inside the thermostat opening that's only 39mm deep. I can't be certain it will hit as it is slightly off centre but since they both work fine, why not put the OEM item back in?
Refilling was done v slowly carefully, and after part filling I ran the engine. I burped the hoses repeatedly, and used the bleed screw to remove any bubbles - over several heat cycles. It totally felt right. However, the test run again showed the gauge hitting six bars at traffic lights after highway speeds - and stay idling long enough it would hit seven bars.
At this point - confidence destroyed - I took it to my local dealer and asked them to have a go (from scratch). They did the same, but they even put the front wheel on a lift to put the rad at the highest point. Everything checked out - they were confident that all air was eliminated. But still - the gauge shows behaviour that I simply don't recall before.
So I'm just going to ride the snot out of it.