is it a big job to change the alternator on a transit van 03 model?

the battery light was on for 2 days, then it wouldnt start, jumped it of a smaller battery (it was in a 206) and it started fine, after some driving tried to jump again with no luck, got it jumped of a vectra then and it started, diving up the road it cut out for no reason, and wouldnt even start with a jump from the vectra, do i need a new alternator, is the vectra battery not big enouth to start it?