We have just had an extensive house renovations (loft, side return, floors, plumbing and rewireing) and had all of our sashes restored and draftproofed, as well as having new bathroom and loft sash windows put in.
I can't recommend enough the guys we used as the most reliable, professional, detailed and fair priced, and trust me - after 10 months of renovations I have had some cowboy tradesmen!
Guys are Polish, called George and Adam and based in Brixton, details on their website http://www.asashrepair.co.uk/
and I have already had people over that are considering to use them to show the work that they have done.
I'm also getting a quote for the staircase and internal doors from them as the 3 painters we had were not good with woodwork.