I don't think the handbook is the clearest explanation, I tried mine today and it works fine. Make sure you firstly set the assist option (in the dash centre status display) from Cruise Control to Speed Limiter. Then at the appropriate speed set the 'cruise control' - except this will now be the speed limiter (and shows LIM on the symbol) and all should be good. You can still change the speed limit like you would using the same cruise control settings. Changing via menu is a faff, previous non Seat vehicles I owned simply had the button on the wheel without having to go through any menus.