I've been happy with modernupvcwindows, they do 3G 4/14/4/14/4 argon fill superspacer, U=0.7. These are replacements for three windows that I enlarged as part of the EWI process, so they've only been there 2 years. I don't know who made the original units that are 30 years old, but they seem fine and I just wanted something similar. I went with the plain white, non embossed, on the basis that it seemed the hardest wearing and could always be sanded a bit if need be.
Thought about replacing the lot, but couldn't bring myself to scrap all the perfectly good stuff.

Once they sent 2G clips rather than the thinner 3G ones (it's the same profile for 2G & 3G, but thinner clips for 3G as the 3G unit is 40mm thick, 2G only 28mm. I emailed them, they couriered the right parts next day, everybody makes mistakes, but they came good!
Only had for a couple of years, but fine so far. Metalwork stiffeners where they should be, all seems robust.

Posted By: revorI have been somewhat disappointed with my upvc windows. My top hung ones leak in strong wind whilst the side hung openers are not locked to the frame whilst the full length the side hung ones are full hinge on hinge side and 3 point lock on the closing stile. Thought might be able to get a fatter replacement seal but can't as the seal is moulded as part of the upvc extrusion. Bad design at end of day. Bad choice on my part. So something to check on.

Do you happen to know the make?