Although there are many DIY tasks you can safely perform at home, unless you are a trained professional, working with gas appliances is not one of them. A Gas Safe registered installer with the required competence to do the particular work should always be used. By law (a) anyone (whether an employer or self-employed) running a business, e.g. a plumber, who carries out work on a gas appliance/fitting must be Gas Safe registered and (b) any individual doing gas work must be competent to do it safely. DIY gas work could be dangerous and is likely to be illegal.