You go directly to your local authority and register the work for building regulations. Your not going to be RIDDOR reporting something in your own house and because its a competent gas fitter doing his own gas work he has no requirement because its not at his place of work.Benchmark is not a legal requirement but to register the boiler with the authorities is. How is this possible with no Gas Safe Registration number! How would you report a RIDDOR if and when needed to!