Brief update, little relevance but highlights Apples after-sales service.
The iMac came back with a mark on the LCD, so they are sending me a new machine:
Machine they are replacing is a late 2012, 3.4 i7, 680MX.
The replacement is being built now and will have the 3.5 i7 & 780M, plus the other benefits of the 2013 model.
You really can't fault that service can you!