If the boiler has no internal trap then this component will not be safe in my opinion as it might vent naughty gas inside the building....which is what all the legislation is about oreventing. centralheatking
Hold on. Is it illegal for him to work on the waste pipe if the boiler does have its own trap? And can you link us to an authoritative website explaining the legislation as this is starting to make me think that this rule means a non Gas-Safe plumber won't be able to work on pretty much any waste pipe if this is the case, given that nearly all waste systems are ultimately interlinked.