As per the HSE website, It is illegal for anyone to use a gas appliance if they suspect it is unsafe. Turn the appliance off and do not touch it until it has been checked by a Gas Safe registered engineer .
If you suspect there is a gas leak you should immediately do the following:

• Call National Grid’s Gas Emergency Freephone number: 0800 111 999
• Open all the doors and windows
• Shut off the gas supply at the meter control valve (if you know where it is)

HSE website recommends the use of CO alarms as one useful precaution to give advance warning of CO in a property. Importantly alarms should not be regarded as a replacement for regular maintenance and safety checks by a Gas Safe registered engineer . CO alarms cost from £15 and can be purchased in most hardware shops. Before purchasing a CO alarm, always ensure it complies with British Standard EN 50291 and carries a British or European approval mark, such as a Kitemark. CO alarms should be installed and maintained in line with the manufacturer’s instructions

Carbon monoxide (CO) is a colourless, odourless, tasteless, poisonous gas produced by incomplete burning of carbon-based fuels, including gas, oil, wood and coal.
Symptoms to look out for include:
• headaches or dizziness
• breathlessness
• nausea
• loss of consciousness
• tiredness
• pains in the chest or stomach
• erratic behaviour
• visual problems
For more information visit the NHS  or telephone NHS Direct on 0845 4647.