Sounds as if you were running in gear at idle rather than in true HDC. HDC works to control the speed if it goes above 5 mph in 1st or more in 2nd (which you can then adjust using the +/- cruise buttons). You need to be on an incline that would make the vehicle speed go above the "set point" for HDC to hold you back, you cannot use the cruise buttons to adjust the idle speed. Gone... 2013 XS TD4 Manual Firenze Red
Gone... 2008 (09MY) XS Td4 Manual Stornaway Grey
Prior... 2003 G4 Discovery 2, 1998 Disco 1 and 1997 Disco 1 Company car before that
Also gone in Nov'13, after 7.5 yrs, to new owners with time for more TLC - 1980 Series III 88
Current 2015 Discovery Sport HSE 2.0

Makes sense.
Laziness factor in traffic jams abandoned(unless they're downhill)