UK Car Repair: expansion tank on megane overflowing, expansion tank, hosepipe

Question
Everytime i fill up expansion tank, it overflows when engine gets warm.The car does not overheat,but no hot air blows.The oil is clean, I have noticed the seal on the cap for the expansion tank seems to be coming away.The engine idles nice and smooth.No steam from exhaust. Should I try k seal,change the cap or bleed the system.The car is 98 1.6  megane.

Answer
If the cap leaks then do replace it, it is an important part of the system.
If you get no hot air you either have a blockage or airlock at the heater matrix, wait till you have a good cap on the expansion tank before you replace anything or do anything at all...
but reverse flushing the matrix with a hosepipe would be my first port of call once the system holds pressure and the heater still does not work.



---EDIT---

an oversight...

You are only filling the expansion tank to the FULL mark, and not to the top?...
The expansion tank is there for when the water expands, it needs to be filled only halfway or so to allow for this expansion - it doesn't matter if you overfill it, the excess will go through the overflow, it's self levelling like this.