£15000 has got to be a laugh. I have 4 sash windows on the front of my house - each one is as big as a patio door and they have a complex and rather fetching design. They are the original units - the house was built in 1895. I've been quoted around £10000 to replace all of them with modern double glazed units that retain the original features/shapes.
Anyway listen to what they say to you and if they don't agree to your reasonable demands ask them who in the organisation you have to sue. Get a name and if all else fails either fill in a small claims court summons request or if the amount