I need a new back door

[Culebra]

Thinking of getting a glazed PVC door but as double glazing sales have historically had dubious reputation, I am a bit scared of choosing a company just from an advert or flyer. Last time I tentatively rang one up I got a pushy bloke and repeat calls which put me right off.

Can anyone recommend a national company that did a decent job for a decent price? And names you would avoid?