Coolant issue

Hi all, ever since I changed the coolant I've had nothing but issues.
I bled the system and the car ran fine for a few days then all of a sudden at a set of lights temp went up and smoke started coming out, later found a coolant leak near the thermostat metal housing onto the cylinder head, managed to seal the leak with some sealant I have, the car runs fine still got power etc and no smoke, however the temp gauge won't move at all after driving for about 15-20mins, I'm worried as I don't want to damage the cylinder head and end up with a nice repair bill! Any easy ways to tell If the coolant Is doing its job and still keeping the engine at a good temp?
Cheers josh (by the way there's no discolouration in the coolant still red and not any loss)

It sounds like you have an air lock.

Fix leaks by replacing the gasket, seal or worn part, sealants are a very temporary solution.

The C3 takes a special coolant and there is a topic on this coolant and how to refill the system without air locks in this C3 coolant topic (here)