Kevin,
You either have plastic windows fitted at ££££££ cost or save money by restoring the original wood - I did it a year ago.
These Youtube vids shows the basics...
http://www.youtube.com/watch?v=D9t-yMmGgSI
http://www.youtube.com/watch?v=OBobNs52b0k&feature=related
http://www.youtube.com/watch?v=3g9m5H3XCZU&feature=related
The product set is described by the setting time 1 - 4 - 16 hours (I used 16) There are two applications a primer sealer in two parts and a filler fixer in two parts - Dryseal and Dryflex respectively. You dig out ALL the rotten and damp wood - brush on the primer sealer - wait - mix up the filler slap it into the voids - press on the new window sill, smooth off the resin and decorate the next day.
It's very sticky and vinyl gloves are essential. Set up costs may appear expensive but well worth it in my view. Mix up on any old sheet of clean plastic or perspex and clean up with meths. You will need the product set itself, a dual-tube dispensing gun and a set of spatulas.
I got my Dryflex from a Dulux Trade Centre - I repaired about 40 panes and perhaps 10 metres of window sill for about £200.
The literature suggests a gap of about 5mm between components so there's no need to be particularly accurate cutting the timber. I used a router bit in an electric drill to dig out the damp wood.
Superb stuff... Good luck...
Peccavi
September 2009