I still question the integrity of the battery, and a weak battery can cause ALL sorts of issues in these cars. Let's discount what the PO said. Two ways to properly test the battery:

1) Quickly: Attach the car to a sophisticated digital load tester like the Snap-on EECS500. These can test the battery immediately. I'm not a big fan of the old fashioned load tester for a giant battery like the X5 has.

2) Slow way: Charge overnight. Remove the charger for several hours to remove any surface charge, and then test the battery with a volt meter. If you don't get 12.75 volts or so, the battery is likely bad. This is not as accurate a test as option (1), and takes a lot longer, but it should give you a general idea.

Thanks guys, really appreciate the feedback. The battery is now on charge overnight, I shall report back tomorrow with the voltage readings. I'll be peed off if the battery is at fault given the PO said he had it checked by 3 different mecahnics... he never mentioned having the alternator checked so that will be the next thing to check after the battery.

You can check the battery voltage and the alternator output by unlocking the cluster and going to test 9. Don't know about Australia but most auto parts stores here will load test the battery for free.

It's also possible the PO used the wrong battery. These cars use a lot of juice, and need the proper, and quite large, and costly, battery. Not a place to cut corners. I do concur with trader4's comments as well. Lot's of hack mechanics/autoparts stores will stick any battery in the car with the terminals in the right place. That's simply NOT good enough for a BMW. And with the later cars, (E70 and beyond), the new battery needs to be coded to the car.

I think the battery is the original one! It's the one showing the green, black light.. It's huge!

So what's the voltage after charging and letting it sit off the charger for several hours?

BTW, if you do this while the battery is in the car and connected, and you get a less than satisfactory result, there's no guarantee you don't have some sort of parasitic current drain. If the result is good, you're good. But if it's bad, you might want to repeat the test with the battery disconnected just to be sure you're not condemning a good, (and expensive,) battery.

Or just have the battery load tested.

I've had the battery on a trickle charger for the past 2 days - it's still charging! Once it's fully charged i'll report back.