tenspeed

It sounds like the bottle has a magnetic reed switch to monitor the coolant level. On a Pontiac forum that I was on, the fix was to give the bottle a sharp rap with a screwdriver handle. Percussion maintenance can be crude but sometimes effective. It sometimes works on starter motors and fuel pumps.

shamus_mcd

So a wee update i ran the car with cap off and sitting nose up on a hill and half on a kerb as i has similar issue with a car many moons ago.
After 5 ten mins i got a glug and a whoosh and water bottle level dropped.
Must have been an air lock.
Filled up to max on the level when cool.
So far no warnings and no loss of coolant.
Fingers crossed.