The sill on our bay window has gone rotten and needs replacing. Can anyone offer advice on how to splice the sill and replace the rotten timber only rather than the whole window? Or any books that show how to do this?

Thanks

The rot is from condensation or a leak so that needs to be corrected during the repair. Start by removing the trim with a flat bar. Not knowing how extensive the rot you just need to prying off all bad lumber. If this is a leak the rot is likely deeper than you might have imagined.

I assume your talking about the exterior sill of the window. If this is true you want be able to pry this off unless it's a 2 piece sill, what I've done in the past is go to Lowe's or home depot and buy a new sill, I think they come in 6 and 8ft pieces. What you'll need to do is remove the rotten sill. As far as doing this you need to remove all the rotten wood from the sill, I've used a chisel and skill saw before, most likely you may need to remove a couple inches of sill all the way across the window,

Use the skill saw to cut as much as you can across the window sill, as far as a degree I think it is a 10 degree cut, is the saw want fit use a jigsaw. What you can't cut use a chisel to clean up the rest of what you need to remove. If the finish cut is a little rough then use a hand plain or electric planner to smooth it all out. When you reattach the new sill I like to drop it down 3/16 to a 1/4 to make a caulk joint. If you try and make it join back up flush the joint will have a crack at some point where water will get in and start the hole process over,
As far as attaching the new sill, we glue it heavly and use 10 or 16 penny hand drive nails, guns nails just are to small and to short in my opinion,
Good luck, time wise and depending on your experience and tools you may a be looking a 4 to 5 hours