"(1) An employee who is dismissed shall be regarded for the purposes of this Part as unfairly dismissed if the reason ……is that - (d) in circumstances of danger which the employee reasonably believed to be serious and imminent and which he could not reasonably have been expected to avert, he left (or proposed to leave) or (while the danger persisted) refused to return to his place of work, or any dangerous part of his place of work"