From the 1st of April 2009, the “Gas Safe Register” replaced the CORGI Register. All gas safe registered engineers must carry additional commercial gas qualifications for the installation, service and the maintenance of equipment in any commercial premises where gas pipes, boilers, cookers, heaters and meters are operated.
You can check this by calling the Gas Safe Register before you get your work completed, in order to verify the registration of the engineer. Domestic gas engineers who don't hold the gas safe qualification are also breaking the law by carrying out commercial work.

It is a criminal offence to hire an engineer who does not have the CODNCO1 qualification.

Work that is deemed commercial must be worked on by a commercial qualified gas safe registered engineer include the following:

Any boiler or gas appliance that has an output capacity of more than 70kw  
Gas installations with a volume of .035 Cu Metres or higher
Any gas pipe that is larger than 35mm in diamaterm, Any commercial premises which contains commercial appliances (including all commercial kitchens, restaurants, takeaways, hospitals and schools), and most buildings with shared heating.