According to the Energy Saving Trust 60% of domestic CO2 emissions are accounted for by boilers. If your boiler is over 15 years old one of the most effective ways of reducing your energy bills is to replace an old or inefficient boiler.
Modern condensing boilers use approximately 60%-70% of the gas compared to an old boiler. Unfortunately, replacing your boiler is both disruptive and expensive. However, if your boiler is old and inefficient it is possible to recoup the costs within 5 years through lower energy bills. Replacing a boiler is not a job that can be done by anyone.
The installer should be CORGI registered. Many plumbers are Corgi registered these days [search for a local plumber] is Corgi registered. You should ask to see their certificate