Hi all, Sorry for the delay in responding. The plumber has now run a second return so the upstairs and downstairs returns now have there own returns and these then combine together to go back to the boiler. However the problem persists with the upstairs still heating up via returns when downstairs heating is on. I’m not sure if he had a clue what the issue is. He’s also altered the pipe work around the boiler, bit to no avail.