HI. I am thinking of buying a property. It is a 2 bed terrace, the second bedroom has been converted into an attic room. In the bedroom there are three small double glazed windows. However, the window sill has a very obvious bump in the middle (the windo sill goes up in the middle - like an upside down smile shape!). Any ideas what has caused this? How i would mend it so the window sill is straight - and the cost and effort required???!!! I am a first time buyer with little diy knowledge!