DON'T EVEN THINK ABOUT IT !!!
Gas is potentially extremely dangerous. You should NOT attempt any job that relates to gas or gas appliances. You need to get a fully qualified gas installer
By law, you must be on the GAS SAFE REGISTER to carry out gas work in Great Britain and the Isle of Man. The Gas safety register supercedes CORGI who used to be the governing body.
Check out the GAS SAFE REGISTER website which gives loads more information and a useful gas safe installer in your area.