Steve, PLEASE don't listen to the above and give up! Seek legal advice, if they say it's a non-starter then fine, but don't rely on people with no legal experience on a forum to put you off even trying, it would be worth checking if you get free legal advice through your bank account or credit cards etc. This seriously sounds like negligence to me and could have resulted in a much worse situation, imagine if this had happened on the road and you hit a child on a pavement!
After that, I'd probably go to the garage and *calmly* point the problem out to the manager and see what he says, I'd be interested to know Mark@Abbey's opinion on this, if Abbey did something like that I would have thought they'd be absolutely devastated! Then possible go the Ricey route for the fun of it.