Battery - alternator?

hi there, iv had my vectra 98, 1.6 for a year, iv only had minor fualts, but 3 days ago, tried to start it, but car completety not working, none of the dashboard lights do not light up. So took it to mechanics, and they said i needed new battery, which i purchased. car was running fine, but the next day, same thing happened? so i kalled out the RAC, and he said that it might be a electric problem, he said when the car is off, something is eating up the battery?, they tested the alternator., it was not brillioaint, but they said it was okay, therefore im very puzzled on what to do?i took it to the electrik mechanics, and they quoted me aprox £400, i mean come on... they said something regarss to computer quad is not working. neways could u get bak to me on what i should b lookin at.. thank you.