If there is a gas appliance in the room it might be for ventilation for
that, in which case it could be dangerous to block it up. If there is no gas
appliance in the room it should be safe to block it up

Hmm. No gas appliance but we have a "real" fireplace with an open
chimney. (although have to use smokeless fuel).

Actually that's a point. Do chimneys let out huge amounts of heat too?