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.

I tried the test light idea and found that the sterio was not earthing properly. I also got the alternater checked by another electrician and he said it was ok. He checked it separately, and I watched him do it so I think that we can say that it is good. The battery is still going flat, but it could be that another battery has finally given up. I am getting the battery checked atm. Hopefully the bad earth is the cause of the problem.
I'll get back to u later on the results

Thankyou for ur replies.
As for the problem being a faulty battery. I am pretty much 100% sure it is not. This is my 4th or 5th NEW battery I've put in this car and they shouldn't all be faulty.
Hopefully I can find a GOOD auto-electrician to help coz the ones I've seen so far have all said there is nothing wrong. Being a female also makes it harder to convince them that there is.
Anyway, I shall look into the rectifier idea. Is that a part u can buy separate or is a new alternator needed?

Hi. My B310, A12 has a problem with the battery going flat. My neighbour had a battery problem with her stanza and said changing the alternator fixed it. I checked the alternator today. It was putting out 14V max. when it first started, but a few minutes later it only put out 13.something at idle and did not change if I reved the engine. The alternator has had the bearings and brushes replaced. Could it be that the rectifierin it is not working properly. Has anyone else had this problem or know how to solove it?