Firstly I would like to thank everyone for their advice and help over the past few days. I have been to Apple store who have agreed to fix for free!
Basically they examined the iMac and found no damage whatsoever elsewhere. The guy said that it was an unusual crack and was not consistent with accidental damage. At some point either in manufacture, assembly, packaging, delivery or the week I had it something happened to weaken it. What, probably never know. They were prepared to believe my version of events and fix for free!
Moral of the story, honesty is the best policy. Apple, amazing products and amazing customer service. Thank you.