Have you got the Hi Line with SatNav? If it doesn't shut down properly it can lead to battery drain.
If you PM Big Frank he will be able to help with an alternator but that may not be your problem, something is draining the battery unless you are sure that the battery isn't charging. You should be able to check this by checking the state of the battery after a long run, if it's fully charged then the alternator is working properly, if the fully charged battery drains overnight then you have a short somewhere which could be the SatNav not shutting down properly.
No, I couldn't get to POL.
Nich.