Alternator Or Battery?

Can anyone help me with this problem that happened this evening. Was travelling from our cousins caravan 15 miles away and about 3/4 of the way home we had to suddenly stop at a set of traffic lights. The cars power and engine cut out instantly. We managed to get a jump start and got a few streets away and the same thing again. We bought teh car 2 weeks before and until tonight there were no problems what so ever. Got another jumo start and managed to get back home. When putting the lights on back home the car cut out again and when trying to start it again there was nothing...a click but nothing else. could it be just battery or most probably the alternator.