from my receding memories of school I seemed to remember a teacher once saying about putting "an" before a word beginning with "h". Anyway I've just looked on answers.com and was directed to a useful learning english site:
We know you use "a" when the noun you are referring to begins with a consonant and we use "an" when the noun you are referring to begins with a vowel.
This is what they say on the matter:
Pronunciation changes this rule
If the next word begins with a consonant sound when we say it, for example, "university" then we use a. If the next word begins with a vowel sound when we say it, for example "hour" then we use an.
We say "university" with a "y" sound at the beginning as though it were spelt "youniversity".
So, "a university" IS correct.
We say "hour" with a silent h as though it were spelt "our".
So, "an hour" IS correct.
I'm not sure if this explains your - "an S.A.S. paratrooper"
Has anyone else any ideas on this?