I'm 27, and there is no way in hell I would have a 13yr old girl back to my flat alone, unless she was a relative.

There is no need to put yourself in that position to begin with. Either he asked her back to his house, which is seriously dodgy, or she invited herself, and he said it was ok, which is seriously dodgy.

She didn't break in. Why put yourself in a position where an accusation can be made, never mind a sex act performed?