The great irony of uPVC windows is that for all the benefits that uPVC windows bring to a house they should sell themselves. Unfortunately, competition to sell uPVC windows and doors is stiff and companies will employ sales reps on zero basic salaries who only get well paid if they massively over charge the customer. Many firms give reps 60% of any sale they get over the normal price. This system means that reps can only get a decent salary if they rip you off.
This is something worth keeping in mind if you are looking to buy uPVC windows. How to deal with uPVC windows sales reps is an article full of useful advice about how to get a good price from a rep. In many cases uPVC or vinyl windows companies will not give you an on-line quotation unless you fill in a form stating your telephone number and address. They obviously don't want to give you an online quote, they want to send around a rep who will bore you silly with lies for two or more hours. Alarm bells should start ringing if your first refusal results in a 50% price reduction.
Always get several quotes from reputable national and local firms and ask friends, family and neighbours if they can recommend a decent uPVC supplier.