possible alternator failure?

Hi, wonder if you can give me any advice, i have a chrysler voyager 2.8 CRD LX on a 2004 plate, noticed just after i got it that on a few occasions it was difficult to start, and lost power twice whilst driving but thought nothing of it as it was a new car and didnt want to go back to dealer complaining, so bought a new battery and all seemed well. About 2 months ago i drove to the shop, was in there about 5 mins when i came out the car wouldnt start so i called breakdown service out and they told me my battery was dead. They boosted it and it worked fine for 2 days until i got a new battery. Had no problems since then, until the other morning i went to take kids to school and it wouldnt start, lights were very faint on the dash and the speedo and rev counter went haywire when i tried starting it. I bought a power pack just incase id left lights or something silly on though im pretty sure i didnt, left it on charge all day and when i went to pick kids up it worked fine started immediately, ten mins down the road i lost all power, speedo wouldnt work nor stereo, lights etc. I managed to coast home where it died on the driveway and hasnt started since. Im not happy as my warranty has only just run out and ive spent nearly £900 on it for service, first mot, brake pads, discs, tyres etc so really worried its something major and i havent got a clue when it comes to under the bonnet. Also could this be related to the problems i had just after i bought it, if it is the alternator does it go immediately or can it happen over time? Im not sure if im better off sticking to the chrysler dealer for repairs or using a regular garage, what would be best? Many thanks for any comments, much appreciated :)