First things first. Leave the dent alone if it is only .5 mm deep. Not worth fixing. A cats are lightly skinned boats, so they will surely dent if you smack em hard enough. as far as repairs go, I would not trust a normal fiberglass guy repairing them. Find out who repairs high tech dingys if you are going to do it. the gelcoat on an A is very thin and easy to sand through.
If you want to try a repair, first try heating the area gently with a heat gun. You may sucessfully make the foam core pop back to the original thickness. I have made several dents disapear by doing this.

good luck,
Eric