Hi I need some advice on the below.
when I am doing about 40 mph I press the + button the light comes on on the dash to show cc is working. After several presses it sets the speed.

but if I'm on the motorway at 70 mph and press it. The dash light comes on but as soon as I take my foot of the accelerater the speed starts to fall and continues to fall. Even if I press +several times nothing happens.

any advice as it is driving me mad.

mine does this if i have been in a 50 limit on motorway and speed up to 70. the ecu thinks i want to do 50. if i press cancel before going up to 70 it works ok

I only have to press mine once and the speed is set.

valks - you failed to say what model you have. Do the good thing and update your profile so that your model details are shown to the left of your post (as mine are). (hover your mouse pointer over the car icon)

The cruise control should be "on" as soon as you press the "+". There is a minimum speed but it should be working at <30 mph and up no problem. Additional presses should just increase the set speed. Make sure you are pressing the "+" firmly. To prove it is working, if you hold it down you should be able to make the car accelerate. If this does not work, you may have something as simple as a switch problem.

Bob 10MY (Sept 09) TD4 HSE Auto in Stornoway Grey (Now Gone)

08 FL2 TD4 SE Manual in Rimini Red (Now Gone)

Dunkely how is this.

Surely the button is working, as soon as I press the + button the symbol comes on the dash.
is there any way of removing the switch and cleaning it up ?

Valks - found this in the manual:

Cruise control is not available in HDC, or when a Terrain Response special program has been selected.

Anything here connected with your problem? MY2010 TD4 HSE Santorini Black, 19" Alloys, Almond Leather, Lux Pack, Rear Spoiler, Adaptive Xenons

Thanks date.
No sorry I do not have any of these items selected.

Valks - Yes, your icon works now!

Re your CC, I agree, if the "+" switch operates to turn on the dash light, the switch itself may be ok. Looking at the circuitry it will likely need a diagnosis by the dealer. Wiring goes through the steering wheel module; CCM; ECM, etc. Not easy for those with a basic lecky brain like me to get to grips with Needs an electronics wizz like Alex to read this.......

Bob 10MY (Sept 09) TD4 HSE Auto in Stornoway Grey (Now Gone)

08 FL2 TD4 SE Manual in Rimini Red (Now Gone)

Is Alex able to advise?

My cc is definitely not working at speeds over 50 mph.

Sorry, now I've seen the thread...

When you set the speed, keep your foot off any pedal. It's important that the pedals are not touched when you set the speed.
As dunkley201 said, keep the + button pressed until you feel a slight acceleration. Only then release it. The car should now keep its set speed.
A press of the throttle will accelerate the car, but when the accelerator is released, the car will return to previous set speed.
A press on the brake will disable the system up until resume is pressed.

So if I'm driving at 70.
I have to take my foot off the accelerator and then press + button.

is this correct ?

As I thought you had to hold the speed steady.

Just hold the throttle steady. Do not touch brake (or clutch). 10MY (Sept 09) TD4 HSE Auto in Stornoway Grey (Now Gone)

08 FL2 TD4 SE Manual in Rimini Red (Now Gone)

No this is not working.
At 70 if I hold the speed steady then press the + button the Icon comes on but if I take my foot off the accelerator the speed starts to fall.
even if I hold the + button for 5 seconds.

But under 50 this does work.

Does anyone know what the most common faults are on the cruise control ?

This is how it was from the beginning?