I don't think this is right: The temp gauge arm only ever rises above the bottom marker if left in traffic. Then the reads exactly midway with the arm at 3 o'clock. When moving it drops right back so it's almost touching the 50 deg mark.

I pressume this is completely wrong, in fact when I first saw this I thought the thermostat had been removed. However it not the cartridge type, and the thermostat housing is definitely there. I guess that means it's failed.

Any thoughts

Had exactly the same concern 10 years ago when I first got the barchetta.
Ordered a brand new thermostat as I came to same conclusion as you; this one must be **%ed.
Changed it. Know what? Didn't make a scrap of difference. Think they just read low!

Well, that's sort of good news and bad news. Good, because we may not have to replace the thermostat. Bad, because the fuel consumption can't be blamed on the thermostat if it isn't faulty.

They all read exactly the same. Sorry about your fuel consumption