Is there any way, e.g. using thermal camera, or any type of specialist, who is able to work out the real-life U-value of a specific element, e.g. a glazing facade/window?

We had a poorly manufactured and installed glazing facade installed last year, and are currently in process of making a claim against them. They claimed a U value of 1.3, but they left such 3-4cm large gaps around the glazing (which they then filled with foam) that I am sure it's not 1.3 in reality.

Yes - I have had good results with an IR thermometer- they tend to have better stability than an IR camera. Measure the indoor air (Ti), outside air (To), and indoor glass temperature (Tg). It’s then a ratio of U values, and we know the still air boundary U=7.7W/degC/W^2.

So Ug=7.7 * (Ti - Tg) / (Ti - To)

When measuring the temperatures, use the same IR thermometer, point it at a piece of paper inside to measure indoor air, and stick a label on the window to get a good temperature of it. We want outside air, not ground or night sky, so again measure something lightweight. It’s best to do it when it is cold out, and get all the measurements at the same time so the thermometer doesn’t drift much.

If it’s 20C in, 0C out, I think a U value of +-0.3 is plausible accuracy, easily enough to determine ‘modern dg glass’ versus old.

It'll also depend on the exact wording of whatever they claimed. Was it for the glazing or for the whole facade or what? And is it a specific guarantee of performance or just marketing waffle?