A couple of years + ago i had the same fault keep appearing, then my battery gave up the ghost, I replaced the battery, and lo and behold the fault disappeared. Wind on 2 years and the fault came back, I realised I was only doing short journeys with lights, heater, air con and sometimes wipers, I also jump started a jag a couple of times. Switched all power drains off, picked a nice day and went out for a 50 mile run. what do you know the fault disappeared. Try this before you try any expensive garage solutions!