Is this a good double glazing quote?

Firstly, are companies that offer one price then drop it drastically really to be trusted? It seems a common ploy of the shyster.
As for triple glazing I have read this up a bit & the consensus seems that it is just not worth it as it makes little difference.
I just paid £4600 for 7 windows & 2 doors/windows; they are those enormous windows that span almost whole wall, varying from nearly 3 m wide to 150, & about 1.50 depth. The doors are part of large units that include adjoined window panels. Because of their size & proximity to floor 4 of the windows had to be of strengthened glass.
I tried to research it beforehand but it's not easy. I could see that a neighbour had some done so asked them about it & they said it was fine,(but it turns out they only had 2 windows done) & the firm, which manufactures its own units, had been recommended by a builder.
But the workmanship/finish is not what I would have hoped for. The men were a touch slapdash. As far as I can tell the sealed units are good, but there are all sorts of niggling problems.
The boss has been round today after I emailed him & that was not a pleasant experience. He bullied, blustered & bluffed but agreed that the window locks are not up to scratch & is going to get them changed for different (better quality?) ones, as well as do a few other adjustments.
This is a family firm of many years standing which also belongs to the Consumer Protection Association. I reckon this latter is the only reason he has been as 'cooperative' as he has.
Sorry, this is not very helpful but might serve as a warning that you just cannot be too careful in the double glazing jungle. My own intuition is it's probably better to get a reputable local builder to do it.