Everything goes 50/50 regardless of who's fault it is because the whole stinking insurance industry is corrupt.
So yeah, £700 it is then, unless you can get a quote to fix it for less. But you'll still be hit with a premium hike next year because she will claim against your insurance.
I take it you were stationary when this happened? In which case it's her fault. If you were both moving when it happened, there's a "DWDCA" case against both of you, so, quelle surprise....50/50.
Insurance is effin pointless, but c'est la vie.