Cruise Control does not work?

I've tried engaging this. The Cruise light comes on, but when I reach the desired speed and try to engage CC, it doesn't work, the car just slows down.

There is no unusual feedback from the engine like the engine revs a bit or bogs etc.

The Brake light system is working ok, so I'm quite sure it's not that that is disabling the CC.

I'm a little frustrated that it doesn't work, especially with the long trips.