Can you tell me if you have water dripping outside from the 15mm copper discharge?
It will happen most when you refill the system and heat from cold
If you are unsure then place a sandwich bag over the pipe and tape it on to see if it catches any water
There is no sign of water dripping outside and no sign of water leaking anywhere in the heating system
any thoughts as to the cause of the problem
So far you haven't given an answer and as yet it has not been helpful