Appliance output, flue type, use of room & safety device will also have a bearing on what appliances are allowed where (new and existing)......again, only added here in case anyone else trwls the net for this info in years to come ;-)
In the United Kingdom, Isle of Man and Guernsey, whenever a gas engineer carries out gas work on your property, by law they must be on the Gas Safe Register & thus suitably qualified for that type of work- Regardless of whether money changes hands.
Chestnut is 100% correct.
Building regulations compliance cetificate is required by law and has been for several years now (whether letting or not)
You will still need a gas safety certificate though.