completely dpeendant on the roads as mentioned above.
my findings on a 500 mile round trip from knutsford to glasgow (CC on the way, no CC on the way back)
on the way, with the cruise control i noticed that on downhill sections it would brake gradually to keep speed and have to work harder on uphill sections to stick to the pre-set speed.
on the way back, you can let the car do its own thing on the downhill sections and use no fuel at all (or burn some and build up a bit more momentum!!)
overall, i got an extra 9mpg on the return journey by not using the CC.
not in a landy - but a vauxhall signum but i cant see the theory being different between vehicles.