Yes, you've said that twice now. Never the less, you are NOT in a designated space and so, parked where you shouldn't be, she owes no duty of care to you. As I said though, she does owe it to the others she is blocking in. Perhaps her insurance also says that she parks in a residents car park overnight?
At the end of the day if there's five flats and three spaces then you cannot inevitably guarantee that you will be able to park there overnight. You don't have designated rights to a space.
The answer to the question you initially asked remains "nothing" though. In law there's nothing you can do to force her not to park blocking you in. It would appear it's a private car park and all five car owners have the right to try to park there. No individual has any guarantee of doing so, nor of not being blocked in having done so unless it says something different in your lease?