Vauxhall Meriva battery keeps on going flat

Hello all,

Recently bought a Meriva. As the title says the battery keeps on going flat.
If I charge it up it usually stays charged. Sometimes it holds its charge.
If I use it and then park it overnight, the next day is a toss up if the car starts. Usually not.
I can drive 500 kilometres to visit family, the battery is charged, the next morning it is flat.
Battery is new, alternator is new, starter motor is new.
I checked all the fuses with a multi meter, also under the bonnet, and nothing shows any amp leakage. Done this a couple of times. Perhaps I checked them at a moment that the leakage was not happening.??

Has anybody had this and how can I go about tracing the issue?
Thanks, Co