I'm guessing you have not so much an air lock but rather a coolant system which is running hot, causing air in the system to expand (there will always be some air in there) and pushing coolant into the expansion tank.
Have you checked the actual coolant temperature via the hidden menu? Do a search on here on how to, it'd be interesting to see what temperatures you're reaching.
Next I'd check the fan operation. You mention it feels like it's going to overheat, I'd expect to hear a fan come on at some stage before that.
If the fan works (both stages), check the radiator for sufficient flow. It could be that a previous owner added some sort of leak cure to the coolant and you have a blockage somewhere.
Have the system pressure tested, if it doesn't hold pressure the coolant will boil at a lower temperature, thereby increasing the expansion issue.
Have a close look at the pressure fill cap, they have been known to deteriorate. They're reasonably low cost so it might be as well to throw a new one on. A faulty cap will cause the system to lose pressure but it won't show up with a pressure test as that's usually where the test equipment is connected up.
It might be worth adding a dye to the coolant to aid leak checking. Some of the common leak areas such as the lower corners of the radiator are difficult to spot without.
Lastly, don't forget to update this thread with your findings and hopefully the cure you found.
Thanks for the reply!
I forgot to mention in my last post that the heater blows cold when stuck in traffic, if that matters haha.
Ahh okay. Ive seen about the secret menu on here before, do I let the car get up to temperature first then go onto the menu or do I just do it cold?
Fan operation is fine, when I'm stuck on the M25 the low speed comes on after idling for a little while and the fast one hasnt come on like it used to (used to stay on fr ages after I would park up) so that all seems fine.
I had a leak and used Radweld to plug it, it stopped the leak but could this be the problem now? Will a pressure test show all of these things up? I will have one done anyway, but just want to make sure things are missed because it is just worrying at the moment that it is going to go wrong again.
I have had a new cap and have a spare, my last one warped and let the pressure drop/leak constantly. So pressure cap is fine.
No leaks in the radiator, I noticed one before and hence the previous use of radweld which might come back to haunt me now!
Mainly, I wanna know whether the car is fine running at the moment/will it overheat again? Really, minus my own worrying, it hasnt shown any signs it will overheat, but will is it really a "problem" at the moment that the expansion tank is overflowing?
Thanks for your help.