We have just ripped some of the WSW windows out, as they were so bad (drafts and noise). Used a company called www.mumfordwood.com
who were recommended by my builder.
They were not cheap, but the quality difference between the Wansdworth sash windows are Mumford windows is chalk and cheese. Mumford are very nice windows.
Please don't get caught into the WSW 'nice/efficient' pre-sale (and their nice looking windows in their showroom) . Your windows won't necessarily be like them and you will struggle to get any recompense.