I had a lot of these in my previous house. I found the best thing was to remove them and fill the opening with a plywood blank, giving me time (days) to work on the windows indoors. I soon found that I needed to - very carefully - remove the glazed part, knock the frame apart and rebuild as necessary. Two-part filler after wood hardener is all very well to begin with but doesn't last if used to fill large voids. So, after cutting out all rotten wood, I rebuilt with new wooden inserts where necessary, glued the frame back together, planed or sanded where needed, small remaining gaps filled with two-part filler, sanded again, primed, painted, glazing carefully refitted, puttied in, left to harden, primed and painted, refitted. Then give it a few years and repeat. Solution? uPVC replacements, or sell the house. I did the latter. Divorce helped with the decision, but I appreciate that this may not constitute helpful advice in all situations.