from
http://www.gas-guide.org.uk/emergencies.html
Although carbon monoxide is invisible and difficult to detect, there are ways that you can see whether an appliance (fires, heaters, central heating boilers, water heaters or cookers) may be dangerous. These include
An orange or yellow flame rather than a blue one
National Grid Gas Emergencies (Formally Transco Gas ) - 0800 111 999 (This is a 24 hour emergency line)
Phone transco now better a wasted phone call and feeling a bit daft if the fire's ok then anything else.