Yes it was the Soudal one from screwfix. Totally agree that it's meaningless to give down decibel value without saying how thick it needs to be. Also, they don't specify which frequencies it cuts out. Regarding the latter point at this stage I care a lot less about the low frequencies than the high ones: I don't mind hearing them, I mind hearing their conversations (and their baby).
I stuck a small borehole camera down to have a look in the joists, and it occurred to me that the seal between their wall and their plasterboard ceiling might be poor. With walls never being precisely flat, how do builders avoid gaps around the side of the ceiling??