it really puzzling, the car does not overheat even after running for nearly 1hr. I spoke to BMW technican and he said that they could be a air lock in the system and to try and get rid of the air by unscrewing the little nob next to the cap of the coolant bottle.

I have tried this and notice a lot of bubble and air coming out - the problem i have now is how do i know when the air lock has cleared?

The pressure still builds up when the car is running but it's not as bad as before. I just need to know when the air lock has cleared.