always always use a company who is fensa registered, yes u may save money with a local guy who may have worked for bob the builder etc, however paying that little bit extra saves u alot of worry, you cant sell your house unless you have a fensa certificate or similar.
a couple of very important things to consider when buying double glazing
how long has the company been open?
have you checked them out properly like on companies house, check that they don't repeatedly go out of business
are they a member of a trade association eg guild of master craftsmen or federation of master builders would would mediate if any conflict with work did occur?
do they take a deposit? or stage payments, advice find a company who does not take deposits and pay on completion once you are 100% happy, or a company who takes a very small deposit with deposit insurance
recommendations from freinds and media
seen work they have done
have u had more than 1 quote?
checked out the actual window you will be supplied with VERY IMPORTANT
may sound obvious but it isnt you would be surprised
Its a big decision when considering spending a few grand , but its better to spend a couple hundred pounds extra and a couple of extra weeks making the best decision for you, because the head aches you can end up with can go on and on if its the wrong choice
good luck