1994 Vauxhall Vectra not holding charge

Hi, hope someone can help me, my car started the other cold morning, but I got about 500yds up to end of the road, went to turn right at roundabout and my car died, turned key and nothing not even a groan, I managed to get a jump start and got car home, next day car started first time, I turned off engine and restarted it and nothing again, thought it might be alternator, got a new one had it fitted at home, jump started it again and was fine, was going to go christmas shopping, turned lights on and car died again, please please help, ive still got loads to do for xmas and cant afford to keep getting cabs, thank you maggie