Thank you for your useful comments Tony.
I am well aware of the requirement to use a CORGI registered engineer. By Commisioning works I mean a CORGI engineer will fit. I live on an offshore island and well experienced engineers don't exist, they are more useful for arga and oil fired as gas...
The boiler will fire and run for a short time but then cut out. The fan and pump continue to run then after a minute or so the gas will reignite. This makes running a bath of water impossible as the water runs intermittently hot and cold.
The fault is repeatable with just CH (Winter mode...