More and more? I thought it was dying out.
I think that it's restricted to words in which the H isn't pronounced, like "Hour", or where the transition from H (aspirant) to the next vowel would make pronunciation difficult.
Hotel was originally a French word, so in theory the H wasn't pronounced,so an.. was correct. The Guardian style guide now says: a hotel not "an"
Moreover:
Style GUide
a or an?
Use an before a silent H: an hour, an heir, an honourable man, an honest woman; a hero, a hotel, a historian (but don't change a direct quote if the speaker says, for example, "an historic"). With abbreviations, be guided by pronunciation: eg an LSE student