I must recommend a guy who did a fantastic job repairing several of our previously useless sash windows. He sorted the rotten wood frames, cords, weights, etc and didn't seem to charge too exorbitantly.
He was recomended by both Clapham Builders and our neighbour and the only downside was we had to wait 6 weeks as he is a busy man (all he seems to do is sash window repairs so maybe that's why he is so busy!).
His name is Tuan on 07917 771936.