As in "A HMRC employee" or "An HMRC employee"?
I say "a" because it's followed by a constanant and "an" just sound wrong.
Colleague says "an" because it's followed by a vowel sound.
YANBU - it's "a" obviously. Your Colleague is a (lovely) fool.
YABU - of course it's "an". Were you too busy reading Just 17 to pay attention to your English teacher?