I had a similar problem,I took it a sparky he had one look at the alternator pulley and said the belt is loose.
I said how do you know simple he said the pulley is rusty from the heat of the belt slipping and burning the paint off.
one new belt correctly adjusted and all was well.
You may want to put a rubber mat under the battery I have heard from a few people that leaving a battery on the ground will speed up it going flat same rule may aply sitting directly on the battery tray.
ade

Sorry to all for taking a while to reply to suggestions, but I have uni exams atm . My last one is on thursday, I'll have time to solve this problem.
The battery is still going flat even after the earth to the sterio was fixed. It takes 3 days for it to be struggling to start the car. I really thought that the sterio earth was the problem coz when the car has no sterio it is fine and can sit around for 3 weeks at least. It is not the sterio itself coz I've had two different one's and the problem was there for both. And they were both wired by different people.
The battery is sitting on the tray, so I shall also get a rubber mat to put under the battery.
I'll get back to u all later on the progress.

sounds like its not charging if you check the output it should read approx 14 volts plus / mins 0.5 volts.
The external regulator are mechanical type and are prone to failure. if its too low the battery wont charge and will go flat after a fews days of driving.

I should just add for informational purposes a follow up on this:

- Problem was never sussed out, we never found out what was the direct cause of this BUT I have a theory.

- Battery draining problem no longer exists!

Since we converted the car to manual that is. Note that the sound of the starter motor changed dramatically from a loud whine to a quiet crank (though this may be due to a heavy flywheel instead of the automatic flywheel. Heavy = larger rotational inertia = larger force it puts on the starter).

The battery draining problem may have been due to the way the automatic neutral safety switch works. What it does is has some sort current running to the starter motor which prevents the starter gear from extending (and thus cranking the flywheel) whilst the transmission in and gear other than Park or Neutral. Since installing a manual box this is no longer needed, the current preventing this gear from extending also is no longer needed and so this reduces the needed load on the battery.

Still the only mystery is why this load doesn't show up on an ammeter, though we had someone do that for us and I'm not sure how they checked the load.

So bottom line, those who own Automatic Sunnys with starting issues take this into account, keeping in mind we checked EVERYTHING we possibly could to try and suss it out.