UPDATE
As i previously stated, I was going to get a replacement for this issue so I went and got one 2 days ago.
Apple store Genius notices the light straightaway and replaced it with no question, didn't say anything about it being common but lead me to believe that this was quite common as he didn't act too surprised.
To my surprise when I took the new replacement home the light leakage was identical to my previous one but after using this replacement iPhone for 24 hours I noticed serious battery issues and that it was dropping calls very often (not one dropped the whole week with original iPhone 5 - 8 dropped calls throughout the day on the new one)
The battery was terrible also, drained so quickly so for all these reasons I took it back to get another replacement as i gave up on the idea of a non light-leaking white iPhone 5.
I don't know whether the calls and battery issue was just bad luck as i havn't heard any other compliants so i'll blame that on chance.
The guy that served me at Apple noticed immediatly from my call history and the fact that it was 3pm and my battery was sitting at 50% (charged to 100% and taken off charge at midday)
Thankfully I was picking up my MacBook Pro anyway so the trip wasn't wasted on my part.
He gave me another replacement iPhone 5, this one came from a different box though and didn't seem to have the protective covers on it, yet it was certainly not used before.
I took it home and everything was perfect, battery life and calls. Later that night I noticed that on this one the light is barely noticable, it is still leaking a tiny bit in the same place, but not enough to actually see brightly in pitch black unless you're looking for it. So I guess that i'm happy, I'm guessing that this small issue is very common as i've not yet seen one without it in some degree (seen 4 white iPhone 5's now that all have it in the same place)
Thankfully my light leakage issue is barely noticable but I would still certianly be open to Apple explaining why this is happening at all.
Also, here's a puzzle for you which I can't seem to solve, This replacement iPhone's model number is very strange, On the back it claims to be the standard 'A1429' yet in 'About' it states that it is a 'ND300B'
Any answers to that above would be interesting, Here's an attached screenshot: 
