This was actually the second time I had seen one of these flies on this window within a week. The first time was here.
The main host plant for T. praecox is the field marigold, Calendula arvensis.
Photos taken in Reading, UK, on 2009-07-26.
It might be that this fly was introduced into my daughter's bedroom with some marigold flowers which my wife brought in from the garden and placed in vase.