With this non sailing period my motor is getting it's regular service, pulled the thermostat out, it hadn't closed properly. Bit of a surprise as I thought these were indestructible. Tested it in the pot, not opening or closing so I tried to revive it. Boiled it, froze it, scorched it, with heat gun, that got it open properly, quenched it and finally hammered it. Well a tap really, that got it closed. So after scorching it and quenching a few times it was opening and closing every time. Now the pot test, supposed to start opening at 42d, fully open 52d. Water boiled and still not open. Ah well, another covid19 casualty. So looking at the forums, it appears these thermostats regularly sieze open. Have others had this problem.

Soak it in a cup of white vinegar for several hours. Cleans the salt and corrosion off.

yep the vinegar is the go