I have a victorian fireplace with a modern gas fire installed. The fire place has a hatch at the top to open/close the chimney, which the fitters have connected to a wire to force it to be fully open constantly. However, it loses a lot of heat through this. I believe it is partly ventilated from underneath. How much should this kind of hatch be open? can I close it completely? Can I close it and have a CO monitor? Surely as long as this is OK there is nothing to worry about?
Your flue should always be open when the fireplace is operating. It is a fire and smoke risk to close the flue while the fire is lit.
When not burning, the flue should be closed to prevent heat loss.
In some jurisdictions the damper and top hatch must remain partially open at all times when you have a gas fireplace.
the fitters have connected to a wire to force it to be fully open constantly.
The only reason I'd do that is if it didn't stay open by itself. Even if it seems to, once it gets hot enough it might close. I would not use that fireplace with the wire removed, until I've verified that the flue is cooperative.