Dear Fish

You bought up a good point. The secret that I learnt is that it depends on the sound not the letter. The rule is that there should be an "an" before a vowel SOUND, not necessarily before EVERY vowel.

In your case you should say "A 125 ton block..." becaue the "one" is actually pronounced 'won' which is not a vowel sound. Thats why it would also be correct to say for example " I will teach in a university next year, because university has a "yu" sound at the beginning. So to say "an university" would be incorrect.

I hope the above makes sense.

Ying