Quote:
just got to get through a standard two layer breezeblock wall.


I wouldn't use your standard drill for this, go and hire one along with the correct size of core bit you need, diamond tipped is best. Most hire companies will charge a set fee plus extra for every mm of wear on the bit, but in your case this should be minimal. It is likley that unless you have a very good drill already you could burn out the motor of a standard B&Q hammer type drill.

Drill a pilot hole first through both layers of the wall using a 12mm drill as a minimum (don't use SDS unless you really have to, you want a nice clean exit hole if you can), making sure it's level, then drill form each side with the core bit. You can go the whole way but it could get messy where the bit exits the wall on either side, better to have this on the inside cavity where is can't be seen.

Make sure that the vent is well sealed, as Andy says you really really don't want mositure being pumped into the cavity.

Most important of all, and take it from a man with experience in this (didn't bother checking and flooded a customers ground floor when I hit their central heating pipe!) CHECK FOR PIPES & CABLES. Sorry for shouting that, but the extra 5 mins making sure is well worth it!

Cheers

Cris.