I don't think theres a legal requirement as its Commercial but any reputable company who undertakes this kind of work would issue you a Non Domestestic Gas Form which should show if not all the following, especially with Gas Boilers.
• Business, Job and where applicable, Client details.
• Appliance Details – type, location, make, model & flue type.
• Inspection Details – operating pressure or heat input, combustion analyser reading, correct operation of safety devices, satisfactory provision of ventilation, visual condition of chimney and its termination, flue performance tests (for open-flued: flue flow and spillage tests) and confirmation that the appliance is safe to use, or not as the case may be.
• Meter Installation – accessibility, operability and general safety considerations
• Installation Pipework – labelling/identification, support, means of isolation, sleeving, ventilation when in ducts, as well as whether a gas strength/tightness test has been conducted.
• Details of work carried out – a brief overview of the work carried out.
• Details of remedial work required – a brief overview of and remedial work that is required.
• Safety information – raising and issuing of a Warning/Advice Notice (where applicable), attaching of labels and notification to the responsible person for the installation.
• Declaration – sign off by Gas Safe Registered engineer.
Hope that helps, also make sure they are Gas Safe Registered