I'll try collecting some gas to see if it's a corrosion issue.
The plumber I had used some enormous machine to do the flushing, and according to his bill he did a "Power flush, refilled the system, added inhibitor and tested".
Hi. Not an unusual post I see, but I've done most things I can think of. I spent about £700 this year having a plumber replace a pipe from my boiler to the tank and flush the system completely to eliminate boiler noise I was getting. This worked for about two days after I switched on the heating...