£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 you want is too much for that pay a local solicitor to send them a 'we're going to sue your ar*e' letter. The quicker you follow up on your threat the better it will work. Insurance companies really don't like court cases with domestic policyholders. Especially as it sounds to me like they're trying to pull the wool.
Just out of idle curiosity do you know why you suddenly got subsidence in an old house? Usually old houses have done moving a long time ago unless something happens - water leak, road works nearby or something else that set it off.