A large gap has appeared and is getting larger quite quickly (currently about 1cm wide) between the windowsill on the ground floor and the window. The gap is only on one side of the window. The windowsill is also moving forward. Below the window on the same side where the gap is, there is a hairline crack in the plaster that leads from the bottom corner of the window all the way down. This crack appears both internally and externally. This has not changed shape or size in the eleven years I have owned the house. The house was built around 1800. Does anyone have any experience of this happening, or any advice on what do do?