It should be always“no fix no fee” by common laws in this country, as you and the gas engineer have a contract that he or she is going to fix your boiler with warranty, and you will pay for the fee agreed.
We have been told so many stories by customers that some gas engineers couldn’t fix a boiler problem, but took money, and refused to come back with a lot of excuses. Should these engineers be taken to a claim court they would have no leg to stand on.
There are two types of fees as a customer you might have to pay, regardless a repair outcome. One is callout fee, another is diagnostic fee, if they apply but both of the two should be agreed beforehand.