Maybe your gtd was over sensitive?
That wouldn't explain how neither my GTD nor yours ever saw the warning about a restricted view of the sensor coming northbound through the Tyne Tunnel (but not Southbound). Definitely not working right.
Played around with the cruise settings. Set up a speed on the shortest gap at a speed far higher than the car in front is going. The R gets very close and maintains a gap. Increase the gap with the [/|\] button on the steering wheel and on each increment the R backs off a bit. So the CC bit of it seemingly works when I tried it. But <car>!<car> warnings are few and far between. Something big directly ahead on in the inside lane will trigger the <car>!<car> warning, a normal sized car won't unless i'm 6' off their bumper doing 70mph.
Sometimes the <car>!<car> warning works at a given gap, and sometimes it just goes away when you've not got any further from the car in front, or even when you;ve got a bit closer. It does seem as though my car's radar is working intermittently and perhaps the alert only comes on when it has had a fair few seconds without a response.
A large builders van snuck in a gap ahead of me literally 10 feet off my front bumper and caused a red alert when the car was fully in ahead of me and only after he'd been fully straight ahead a good 2 seconds, about 1/3 of the distance the GTD would've triggered at.
I did wonder if the R had shorter alert distances as it has slightly beefier brakes than the standard GTI and GTI, but 1/3 of the distance?