We lived in the house for several weeks before I built up enough energy to tackle it. But it got to a point where I couldn't bear it any longer and took the bolster chisel to it.
The bolster chisel and hammer took a bit more out of the actual wooden windowsill than I probably would have liked, but oh well!
I used a mouse sander to sand off the old paint underneath all that mirror! And some wood filler to fill in the areas where I had taken chunks out of the wood by accident. I didn't have to be too worried about the colour of the filler because I was going to paint it anyway.
Primer, base coat and two coats of eggshell later, and now its not the worst element of the stairs! The trouble is when you make one part of the room look 100 times better it makes the rest look pants!