Awesome! I have dewormed many a horse, but have always used dewormers with the little ring that spins down to the correct weight - the one I used today didn't, and it shot up so fast that I accidentally gave Maia the whole tube. It was an Invermectin, as I already said, and I did a bit of reading and am not too worried now :P
Thank you all for your replies... I would hate to have ruined my brand new horse!! Gah wouldn't that be lovely? I sound like such a newbie owner, sad hey?