Hello Julian,
Re, Funny how many boiler inspectors don't have a set of stamps with them for the first test! Boiler inspectors should not have any involvement other than to examine the construction of the boiler, witness any testing and record that process and the inspectors findings. The stamping of any identifying marks must remain the responability of the manufacturer or builder.
Ideally in the case of non commercially constructed boilers the serial number of the written scheme document, that being the commencement of an audit trail, should be incorporated in the stamped identification together with the designed maximum working pressure; personally I like to stamp the backplate before fitting to the boiler, the idea of stamping the boiler after completion in it's relatively soft state worries me.