HDC System Fault - New alternator and now new battery

My car has been backwards and forwards to the garage with the following message “HCD system fault”. The special terrains such as sand, snow and ice etc, were restricted and I could not raise or lower the vehicle. Shortly after this I also got the message “low battery”. The battery was tested and found to be working fine. To cut a long story short the dealership decided to replace the alternator (under extended warranty) as they were having problems diagnosing the vehicle. As soon as I drive the car out of the dealership all lights were back on (HDC fault), now they are saying it could be a problem with the can bus wiring? And I need to buy a new battery as this is not covered under the extended warranty and they want £300.00. Any advice? I feel that they do not know what they are doing and I am paying unnecessarily for something which may not solve the problem.