2001 X5 Won't start off own Battery - Help!!!

[wwpxd16]

So I have have a 2001 3L D X5 Sport auto, if it makes any difference. 130,000 miles. I'm having a nightmare with it for the last 6 months and need some help before I commit it to the dealer. Here's what's happening

The car simply refuses to start off its own battery, every morning. Once its jump-started (usually very easily) give it a few mins and its fine for the day, unless you leave a door open or interior light on etc the. It dies within 5 mins.

We are currently having to charge the battery every night, to ensure its good. At first it would go a few days but now its every night without fail. I've been advised that it has some kind of safe mode which kicks in if under a certain volts and that something may be staying on overnight etc. the only thing I can think of is the stereo/sat nav/hand free kit as that's been blocking itself quite a bit (the radio intermittently stops until either the sat Nav talks or the phone is removed from the range).

Anybody have any checks a simpleton can do ? I'm reluctant to put it into the dealer to be told its big bucks.

Yes I've replaced the battery, they also checked it was re-charging and said it was