So! let me get this right.....
You were emptying your shopping from your shopping trolley into your car, at some point your shopping trolley rolled off and bumped into a car which was attempting to park, this parking car while reverse parking then hit your car........ Blooming ek! is that correct ?
The Answer to this!
I suppose as your trolley that was in your presence rolled off and hit the lady's car causing damage, then maybe you should obviously agree to put right the damage to her car as both of you witnessed the accident.
Then with the lady hitting your car with her vehicle, then that should be a normal insurance routine where you would claim through her insurance (get her details and give them to your insurance company and let them do the work).
I don't think either case matters where the accident happened, both parties were there and any witnesses (I think you're both at fault and need to do the right thing where both parties are happy).
Most probably the lady was startled and panicked when your trolley rolled and hit her car, she could easily go down that root with her insurance company if that what has happened.
(Anything on CCTV)..... ?