I've got the same problem.
but i have also been told some fixes.
(i think i have a bit missing)
Firstly does the motorway (cruise) button light up with a orange light when you switch it on ?
If it does then you have power....
Next check that the switch on the brake pedal is making contact with the brake pedal.
And finally, look under the bonnet, behind the nearside (left hand) headlamp there should be a black air tube and a round electrical plug.....
What it attaches to, i don't know, as that is the bit that's missing on mine.
If you do have a look, let me know what it looks like, so i can source one.
everyone tells me that those three things are the most common cause of non functioning cruise control.
hope this helps.
Gary.