Its kind of besides the point now guys. We've moved. Please try not to discourage other people from seeking help in the future by questioning their motives, neither the landlord or we as tenants wanted to be in this situation, both parties felt they were right and couldn't understand why the other couldn't see that. The facts in this case are we wanted the vent to be resized because we were told it was too small, the landlord felt it was unnecessary as it was safe when they had been living there. Both parties believed they were right. The situation changed when we had a Carbon monoxide leak. Since we moved I believe the landlord has taken advice from another engineer the result of which I can't tell you as I said we are no longer living there.
What matters is the situation will be resolved either way before another tenant moves in.
Please don't make light of this situation, there was no need to try and determine who was the bad guy. This thread was created so that I could get some advice from the professionals, I'm happy to say many of you very kind people have been great and offered some brilliant advice. Thank you, and in the nicest possible way I hope we don't have to speak again in the near future. Naomi xx