Hang on a minute.
If you were reversing, and she pulled in behind you she drove into you.
Tell her to get bent and that she needs to fix her own sodding car or go through her insurance.
To be honest, the way you described it, it seems like this is what happened. Surely you're not entirely at fault if this is so?
since your insurance is £400pa you've probably got a sum of no claims and driving years behind you already the hit won't be anywhere near a £700 increase.