Hi patbhoy
I assume you have not been without windows since May and have the first faulty set currently fitted. Unless you know better I would imagine that your original windows have been dumped.
Write a letter
http://www.postoffice.co.uk/mail/uk-signed to the company listing the history of your troubles and stress that as they do not reply to your calls you consider they have failed to honour their contract and demand return of your deposit within 14 days.
If they promise to have another go, only agree if they promise in writing that if the new windows are not satisfactory they will return your deposit.
Best of luck.