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I think there is a lot of confusion here and people dont under stand.
In england if you fit a boiler, you need do the building regs cert which you do via gas safe when registering the appliance. In scotland you dont need to do that.
In terms of what you are asking you fill in the benchmark book when commissioning, so you could say that is your cert but as a separate cert I've never heard of it never mind provided one.
Gotta say ive never heard of a flue certificate.