A bit of an update:
I returned iMac number 3 today for a refund. They tried to convince me to have the current one repaired (it's only 7 days old!) and then suggested I try another one (because I love driving 2 hours each way!). I was then told that having 3 defective units in a row was highly unlikely (even though they acknowledged the yellow tinge problem) which made me feel like they thought I was lying about the whole thing.
Over the last 2 weeks I have been promised twice that a manager will get in contact with me, which hasn't happened. Again today I asked to speak to one in store, and again I was told they are all in a "meeting" and I will get a call back. I've written emails to the store (which I was told was the best way to get in contact with a manager), and haven't received a response, and out of desperation I even wrote one off to sjobs@apple.com. This is seriously frustrating.
I know I can't be reimbursed for all the time and money I have wasted, but this is probably the worst customer service I have ever received, and I would at least like someone to take responsibility and say this should not be happening. Does anyone have any ideas about what I can do from here? Anyone else that I can contact (I'm in Australia btw).
Go back to the store about 30 mins before closing time, sit down and say you wont leave untill this is sorted. Dont be rude to anyone, just be persistant.