There are many big national companies that replace worn wooden windows to shiny PVC. This often involves an innocent appointment being booked for a 'free no-obligation quote' and then a pushy salesman turning up and doing his damnedest not to leave before he has your signature to change every window in the house. I should know-I've been in that industry!
Independents Raven supply a minimum of a single window up to a whole house-worth. My advice would be to measure the window that you want to replace and ring ahead to check that they have a unit of the size that you need in stock, as there is not really such a thing as a standard window size. New patio or French doors needed? They have these here too.
Where Raven come into their own is that they can change locks on existing windows that you have, and at a price that is a fraction of the big window companies. Often the big companies will only be able to, (or only agree to), work on their own brand of windows, whereas this company will look change locks or make repairs to windows from any manufacturer. read more