The recent Coronation Street storyline where Fizz Stape was poisoned by carbon monoxide from a boiler that had been badly fixed by a friend has helped to highlight the importance of using a Gas Safe-registered engineer to install all household gas appliances. Gas Safe engineers have to take exams in order to be qualified to work with gas appliances and are also regularly inspected to ensure their work is up to standard.
There is plenty of information about this on the Gas Safe website, including the symptoms of carbon monoxide poisoning and a comprehensive list of Gas Safe-registered engineers, so you can check that whoever is working on your appliances is on there. Better safe than sorry - gas can be very dangerous and needs to be treated with respect!