Can my gas engineer refuse it's use?

Some customers have had boiler installers and service engineers claim that it's seemingly not safe to use. However they may not be fully aware of the regulations and you can refer them to Table to Diagram 34 in Document J of the Gas Safety Regulations. Document J indicates minimum safe distance of install the cover, depending on your boiler.

The document within the gas safety regulations clearly shows that a flue needs to be at least 75mm below any gutters etc and since this cover is of similar material to that of a gutter/soil pipe but with additional heat protection coating the regulation applies the same. Hence it is perfectly legal to install it so long as it is at least 75mm above the flue to fully allow the hot steam and gases to escape without back venting.

You might also want to point them to the fact that these customers are not only bought and used by customers directly but other independent and corporate gas engineers and companies regularly purchase for install in their customers properties too, as they can see from the many online reviews on sites such as Amazon.