I know of no carpenter that enjoys spicing. Let alone when it's in the dark. On the top of a ladder.
A friend had a sash window where the bottom had rotted out of the top frame. I'm pretty sure I wouldn't have done the job if he wasn't a friend as it was very awkward without a 100% chance of it working. The timing was down to me, as it was the only time we could both fit it in.
I machined the piece in advance, ready to fit. I broke the old piece away and cleaned up all the tenons. I then marked the new piece up, halving joints on either ends with mortices to join in the mullions in the middle. I was a fiddly job at the top of a ladder.
I then glued and screwed the piece in a place and used linseed putty to seal the glass. My putty skills are far from the best but it's so infrequent that I use it now, it will look fine when painted though.
The window should last a few more years now, I've told him it's essential that he paints on to the glass when he paints the window as this is the only way you properly seal a window with putty.
Left home at 7 at night and got back in around 12 - not everyones working day!