Double Glazing

GDB2222

I've never had double glazing before. What should I look for in a quote, and is there any rule of thumb way of knowing whether a quote is reasonable? 

silvercar

Local recommendation is usual.

Specific things to look for are trickle vents and the rating of the glass used. 

Stay away from national firms who spend hours in your home then give a quote 3 times the small firms and offer discounts if you sign up on the day.

GrubbyGirl_2

Also make sure any guarantees are insurance based just in case they go out of business.

WIAWSNB

Yes, go local, based on repute. 
We have three local companies around here, and you could choose one at random. They will be near identical in window quality, installation, and cost.
'Nationals' will typically be twice or more the price.
Last windows we had was a good few years ago now, but I had all three and a national out - the latter to see if they'd changed their methods. Nope.
The three local quotes were near identical, but two seemed to rely on their reputation and were even lack-lustre in justifying their product. It would still have been ok going with them as I knew they were decent. However, the third guy stood out, because I'd asked them all for help in choosing a style for my ol' bungalow. Two said, "whatever you prefer", but the third pondered, recommended, and explained his thoughts - transom height, section size, where to have the openers, size of glazing bars, etc. And it made sense. He got the job, and they were brilliant - even cleared out my exposed cavity of rubble which had been causing a localised damp patch.
10 years on, they are as good as new.
Oh, and the deposit was insurance-backed. Good point by Grubby - insurance backed warranty also sensible, tho' you should know if they are a solid business.
Local local local.
Call them out, and ask about frame thickness, glazing ditto, fittings (locks and hinges) - let them explain it. Then compare them. I suspect they'll be the same. DG windows these days are pretty universally decent unless you go super-cheap.
Are you happy about what actual style you want?

Albermarle

The double glazing part only applies to the glass 'sealed units'.

You need to think about what material you want the frames made from.

Softwood - Cheapest and will need regular painting.
UPVC - Middle price - Good insulation, but not everyone likes the appearance.
Aluminium - Higher price - thinner frames and better for very large openings.
Hardwood - Higher price but traditional appearance with less maintenance and more longevity than softwood.

In all cases the quality of the installation is important.