I took it back to the service centre and gave them an earful. They wouldn't let me speak to a technician, but I demanded that they look at the screenshots to see the battery's strange behaviour when "charging" and they admitted that is is very "strange". They had looked at me sarcastically when I first took it and told them to check out the screenshots, but I know there is no way a technician is going to sit around monitoring the battery's behaviour for hours. If they had bothered listening to me I may not have had to go back a second time. I got a call now from Samsung and they said the problem is with the charger and that they have replaced it. I'll go there tomorrow but I'm afraid to have my hopes too high after my previous experience

The Samsung rep did however imply that if it can't be fixed at all then it may be replaced. I'll keep you updated for the sake of others who may have the same problem.