Another thing you could try is using an imperial hex key that's fractionally larger than the metric one it's supposed to use... you'll need to tap it in with a hammer and you'll only get one shot at it, but it won't make matters any worse.
Umm there's a point, you ARE using metric hex keys, aren't you? Coz imperial ones're fractionaly wrong and they'll strip your bolt heads in no time.
Matt
