Battery keeps going flat

In my sisters 1997 Auto Hiace Super Custom the battery keeps going flat after a couple of days - we have done all the normal things (new batteries, alternator etc etc )and an auto lecky has pinned it down he thinks to the EFI Main relay switch staying engaged but because it is intermittent he cannot say whether it is the relay or it is coming from the computer!

Has anybody else struck a similar problem or have any ideas?

check and check , alternator !