Okay knowledgeable folks.

Here's a new problem for the turbo pig.
Was driving 2 days ago and noticed the water temp hadn't risen at all when suddenly there was a pop and steam everywhere with the temp instantly hitting 120.

Found that the top hose had blown off the header tank, so have put it back on and clipped it in place. But the problem seems to have just shifted, came up too hot then dissolved into a cloud of steam again.
Just put 5L in the header and watched it come straight out the bottom of the tank.

I'm now wondering if there's a downstream blockage (hence no water got to the thermostat to show temp rising) and at first the car blew the top hose off when the pressure got too high, then this time as the hose was secure, the pressure just built until something else gave up, not yet sure if that's a ruptured header tank or not yet till I can take it apart.

Any thoughts guys?

 

You may have had a leak that dropped the level lower than the thermostat creating an air pocket atop the engine. Next step should be to replace the coolant reservoir as it has probably split, then refill with a vacuum filling tool to minimize the possibility of air pockets.

And cross your fingers that you haven't warped a head.