At a distance greater certainty is impossible...
Were I in your situation I would disconnect the fan and run external power to it and check it that way.
It is always possible that the fan is good and that the neutral return path is open circuit but it would be a very strange fault.
My money - based on what you say - is resting on a defective fan motor but that too would be a very strange fault.
Your Gas Safe engineer makes a reasonable guess - which is unusual.
Good Luck...
November 2015
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