Reminds me of a time may years ago before we had laminated windscreens and had my company van windscreen break so got a new one and before I left the trading estate (about 200 yards) the screen broke against and no way would the windscreen repairs take responsibility for it because they said the screens can break at anytime so the boss had to pay for another one.
The windscreen company said the van I was in (Ford Escort) must have been in an accident that warped the aperture of the window, a van that was 2 months old at the time.
So I think you might just have to pay the £50 excess and be done with it and is it worth the hassle for fifty quid.