Thank you for your detailed reply!
Yes, safety is most important! Since his installation, the underground pipe burst and resulted in heavy leak from the unvented cylinder!---No hot water and underground water emerged to the kitchen ground...
And upstairs bathroom tap broke which resulted in...
I had an unvented cylinder (and gas boiler) installed by a Gas Safe Register engineer. He later informed Gas Safe Register which issued me a gas certificate. But Gas Safe Register told me I should have a Building Regulation Certificate for unvented cylinder installation.
But the gas engineer...