Morning Les,
-40C is an open circuit reading.
Check the engine harness isn't rubbing through where it passes the starter motor / rear of fan pack.
I suspect you may find green snot oozing out.
Did you fit a genuine temp sender & was there any coolant in the temp sender plug.
I don't think it applies to the BKC but some VAG engines have more than one temp sender - G62 is the sender on the engine.