Battery on 06 Espace keeps going flat

Hi I have an 06 Espace 1.9dci which has had an engine rebuild 11000 miles ago. Included in that was a new alternator and I have just replaced the battery. But the battery goes flat if I dont use it for about 3 days , albeit it goes flat just enough that I have to jump it from my Peugeot 106, which is not too much of a faf but annoying. Does anyone have an idea what could be causing this. I have noticed one or two reviews on the Espace that report similar happenings