It is not stated in law how the assets should be divided at divorce - if it were life would be much more simple but grossly unfair in many cases.
It shouldn't necessarily be split down the middle. If one has put a lot more money in they should be entitled to more of the pot.
One thing is for sure - if you can decide it amicably and keep it out of the hands of solicitors, you will both come out of it a lot better off.
Not knowing the circumstances - how long youve been married, whether you are both in work, how long she had owned the flat for, children and so on makes it difficult, but looking at it coldly you should both get back what you took in, and divide remaining assets equally.