I just had an interesting thought, at least to me anyway.
Victoria Lynn is correct in writing that the article "an" is used before words beginning with "h" when the "h" is silent. But I noticed that you (Rosemay) are from the UK and now living in New Zealand.
I believe you may pronounce more words with silent "h"s than Americans. For example, when Americans speak the word "hallucination" the "h" sound is heard, but perhaps for some people with a British accent the "h" may be silent (but I'm not sure). Therefore, if you speak with an accent where the "h" is silent in "hallucination" then it would be correct for you to use the article "an" in front of the word "hallucination."
But I see you asked the question of an international editor, htodd, so I would be interested to see her response. I am curious if the rules differ when writing for an international audience.