AUDI A3 - WON'T START?

Hi, if i don't drive my audi a3 (Y reg) in any 24 hours, when I go to start the engine, my car won't start, it's being drained from somewhere (i think) it's got a new battery and I have had it to both a garage and an auto electrician who both say there is nothing wrong with it. it's so frustrating as obviously there is. please help.