hi mate,
when i had the car the main wire connected to the alternator came off, its right at the front and we managed to get a new washer connector to fit the wire and bolted it back to the alternator but at the time a soldering iron wasnt to hand so it was just sealed with insulating tape, its probably still Ok now but it would be the first thing to check especilay if things have been knocked about when the aux belt tensioer was done.
Also its a new battery so it is very unlikley to be that.
hope your still enjoying it, im totaly bored with my new golf GTi already!
PM me if you get any problems as i still pop back here fairly regularly