I wouldn’t be relying on what the owners say their plumbers say. Gas heaters can be extremely dangerous if not installed or vented correctly. Is it the present situation where some of the flues may be leading into a non-vented roof space, presumably common property? This definitely wouldn’t be in accordance with the standards, the flues should be vented outside the building. If I were you, I’d be getting an expert in ASAP to inspect the installations and give you a report on what is compliant and any rectifications which need to be made.