Its total snake oil, particularly on engines like ours.
The Terraclean stuff does nothing that petrol don't already do every time you run the engine, especially with the ethanol content in modern fuels. The inside of the fuel system - injectors, fuel rail etc are all spotless, kept so by nice filtered fuel, and because our engines use port injection, the fuel spray keeps the ports nice and clean so there's nothing for Terraclean to do anyway. Your engine's ports were perfect btw.
The only places that really get dirty in these engines are the throttle body (which I cleaned) and the crankcase breather system (which I replaced). Terraclean couldn't do anything with those because they're all upstream of the injectors.
Carbon cleaning does apply to direct-injection engines though (petrol and diesel), especially those with exhaust gas recirculation (egr). The oily vapours from the crankcase mix with the hot soot in the recirculated exhaust gas to make horrid sticky tar which settles on everything. As there is no fuel spray in the ports (since it's directly in to the cylinder) the tar builds up until the engine can barely breathe. Its a common problem on all the FSI and TDI engines, especially the D3 S8 and S/RS4. I've seen TDIs with the inlet so clogged there's barely space for a finger in there! The only way to clean that is mechanically - get in there with tools and dig it out. There's no chemical treatment that's safe to put in an engine which can clean that muck.
You're better off spending your £75 on a tank of Tesco Momentum or Shell V-Power and going for a thrash
Edit - I spent far too long writing that
Good point about dino oil Goran. That killed a lot of early 20V engines.