Mark, you know your Dart 18 !
The foam block run from the bow to the mainbeam, halfway between the deck and the keel. By slapping gently with your flat hands, one on either side of the hull, you can feel if the hull skin slaps against the foam. It will do this if it has broken free. The hull is very thin in this area and the foam keeps it stiff. It does not happen often, but I have seen hulls flapping like cardboard.
The standard glassfibre battens break easily and can damage the mainsail and a new set costs stg£146.00 for a set. I would buy foam battens locally.
Check out www.lasersailing.com and www.dart18.com (tech/talk - buying a used Dart)
Dermot.