I have a rotten window frame on a folding concertina  French door.

Obviously the most logical way of repairing such a frame, is to to take the whole door frame out, and repair it on or off site.
This cannot not be done for a variety of reasons.
So now I wish to look at the possibility if an actual repair can be carried out in situ, without removing the frame.
I attach photographs taken from the inside and outside.

The hard part I think is trying to slide the new bottom frame underneath the glass.
Now as the old saying goes, where there is a will there's a way.
I think the beading goes on afterwards, so if that is the case, then in theory there should be no problem with the glass? 

Any thoughts would be greatly appreciated.

Probably not what you want to hear, but that looks a lot worse than just the effected area in the pics. I would look at replacing the entire door section. Probably easier and cheaper in the long run than what you are proposing.

I also doubt you would get anyone to try such a thing effectively with a nice result.

Replace them. 

Put some pants on before taking photos.

3 hours ago, up2you2 said:

So now I wish to look at the possibility if an actual repair can be carried out in situ, without removing the frame.

It is impossible to repair without taking the window out, since the rails are mortised into the stiles and the bottom rail is rotten, or eaten by bugs, as is the bottom of the right hand stile

 

The glass is held in by the half round moulding and paint so you can take that out just by levering those out, they should be nailed in.

I had all those pics on "blow up", scrolled down and saw this pair of <deleted>.

My first bit of advice is to put a pair of kegs on before you take photos in the glass.

I am not going any further, as I cant unsee that anymore.

 

Frame is shot as others have said. You appear to have a termite infestation. 

1 hour ago, tjo o tjim said:

Frame is shot as others have said. You appear to have a termite infestation. 

It’s impossible to tell without prodding the wood but it’s less likely to be termites than fungal rot, as with termites the paint layer will often hold the original shape whereas with rot it will deform the surface.