Double glazing - what to look out for?

SternMusik

I am thinking of replacing the 17 year-old timber double glazed windows in my house with modern ones. (The existing windows are rubbish quality and beginning to fall apart.) I would love to have good quality modern timber windows, or aluminium as second choice, but they are out of budget so I am looking at PVC. I am in the process of getting quotes from at least three local companies (I'm not going near the home improvement chains).

Having never bought windows before, my question is what to look out for in the window spec? I know I have some design decisions to make (e.g. tilt & turn versus standard casement - much prefer tilt & turn). But I have no idea how to judge if a particular company's windows are good quality or not. Are they all pretty much the same?

What kind of questions should I ask? I am thinking

- quality of hinges and handles/warranty on them
- energy performance of the frame/glass
- safety features: how many locking points, window locks etc
- ventilation: how far does the window open? Does it have a night latch position with locking?
- trickle vents: do I have to have them? (I hate trickle vents with a passion. Why produce a super insulated window frame and then punch a hole in it??)
- who is going to install/what qualifications do they have/are they insured/warranty on installation?

All advice much appreciated.