I am having the same problem. I was warned by a long-time Mk2 owner not to try to change out the Thermo switch because the three screws are fragile and prone to breaking. He reports that if that happens "you will have a very bad day". It is easy to connect a lead to the Lucas connector at the Thermo switch and run it to ground in the cabin with an indicator light. I have made a temporary jumper. The car started fine at 25 degrees (F). I run the car until the temperature gauge gets to about 30 degrees (C), stop and turn off the jumper. What a wonderful thing this Forum is!

 

There is an easy fix to that without touching the 3 bolts.

Buy a new Otter switch

Carefully knock back the holding tabs. Do the same to the new switch, fit the new bi metal (invar) strip & plastic part.

Gently tap the holding tabs back on the new guts. & you are done.

 

Thank you Glyn...