Sorry to hear your problems, i had something similar a while ago and in the end changed the battery, which solved the problem, there has been a history of Subaru batteries being rubbish and unable to cope with the demands of the car.
It could be your alarm draining the battery or the alternator not charging it.
I would take it back to the dealers and insist that you want it fixed this time to your satisfaction or you will be writing to Subaru UK voicing your disatisfaction with the quaility of their goods and the service recieved from your dealer.