Left side is my house with white upvc. This upvc window on the right is shot ,and want to change it. The guy who came to size it said it will cost £700 for the window and £300 for the lintel as the lintel is sagging. Assuming its 1930 property it possibly has wooden lintel. Can i change it myself and how. What are those slate type things above the window frame. Please advise and also recommend some good upvc window supplier in northeast part of london.