If you feel uncomfortable at all with a contractor then they are probably not the contractors for you. We have a list of installers which we have worked with for a number of years and have a good working relationship with. Otherwise ask around, find someone you can see the work of and that you have a reference for, there are also a number of find a trader type websites which will have references from local people so you can get a feel for their work. You can check to see if a contractor is gas safe registered here, this will also tell you what they are qualified to do. We would always recommend that you get at least 3 quotes so you can compare prices.