It is often mechanical items such a switches, rather than the electronics, that cause problems. In your situation, I would have thought it worthwhile to check the switches immediately above the clutch and brake pedals. If these are not working properly, the body control computer will not switch on the cruise control lamp on the indicator cluster above the steering wheel and will inhibit the cruise control function.
Access to these can be obtained by removing the plastic stowage box (the one with the diagnostic socket) below the steering wheel. Four plastic tags - one at each corner - have to be prised up to release the stowage box. Remove the small panel with spare fuses attached to it, to the right of the stowage box, so that you can free the diagnostic socket from the box in order to remove it. Once the stowage box is removed you will see the three pedal switches and have good access to them.
Remove one switch connection at a time and check its operation by pressing the appropriate pedal, with a multimeter (on Ohms) connected across its terminals. (If you don't have a multimeter an improvised continuity tester can be made using a torch battery and bulb) Remember, one of the switches on the brake assembly should be 'closed' when the brake pedal is in the 'up' position.
