Thanks for all the suggestions.
Have kept son's bedroom window shut tight since then but not going to be too happy about that if the sun comes out, it'll be like an oven :( Definately looking for an effective CO monitor!
We do know a 'gasman' but as the people haven't moved in yet the gas wotsit is only working occasionally :rolleyes: but definately smelling gas when it does

Trouble is, when the wind's in the right direction there's no smell :rolleyes: but I can't believe that a vent that close to the house and windows
can be legal

I'm dreading making any noise about this, all we've ever had here is trouble with neighbours, and now it looks as if it's going to be the same again, they aren't going to be happy if we complain :( :(