I took a tentative hammer to the windowsill which it transpires was made of chipboard - one of my least favourite materials. As soon as I cracked open the surface of slick gloss paint, the extent of the rot was revealed. Inside the board were worms, woodlice and earwigs, all happily living off the gently rotting wood, which had lost all of its structural integrity.
I decided there and then to remove the entire windowsill. It was extremely easy, and revealed underneath a poorly finished stone surface. Whatever was there originally has long gone - perhaps it was wood which had rotted over time. There are, however, what look like original bricks under the windows (the windows are not original) which may clean up nicely. I suspect that we'll look to rebuild the window sill using quarry tiles or bricks, rather than in wood again.
Carried away by the ease of which the gypsum plaster underneath the window came off, I spent a good morning chipping it off, leaving only the original lime render in patches either side of the window.
This is a job that we had scheduled for the end of April when we plan to move out of the house and into a caravan because it is horrifically dirty work and murder on the lungs. This wall, however, was so damp that it created very little dust.
One pleasant surprise from excavating around the window was that the original oak lintel is still in place and in good condition, with no signs of damp or rocked or insect infestation. You can see that it has a natural split down its length, but this doesn't affect its structural integrity.