Partly rotten window frame

If you're talking about the cill, this can easily be replaced. If it's part of the frame then this can also be replaced. I would suggest this is a job for a local handyman or carpenter, or speak to your local glazing company, they may have suggestions. We frequently carry out partial refurbs to windows/frames, it's a lot more cost effective than replacing the whole thing if you don't need to. Good luck!

Hi not a problem for a painter and decorater that's my trade many thanks Barrie Randell

There is no such thing as a "small job"...they don't exist..! Every job has some complications involved, some are minor and some are more serious and the key to how us professionals overcome them is how we improvise and use all our experience to get the job done in the easiest possible way in the least possible time and at a reasonable price. That's what I do anyway but we are not miracle workers and we aren't clairvoyents either and I don't give any quote on any job as guaranteed until I have actually seen and assessed the work required.

I do this all the time, you can either cut off and replace the sub cill, the bit that sticks out, or the entire cill. Do understand that the actual rot will be at least twice as bad as the visible rot.