I called Apple to complain about the delays (and hopefully get a free upgrade

) on the 30th November. But Apple said that my machine had gone to 'prepared for shipping' - wouldn't it be nice if the UK Apple store could reflect this instead of showing NYS? Logging in to the US store did show the order as PFS.
The replacement shipped on the 1st and landed here on the 3rd of December, covering half the world in three days.
It works - hooray!
Until Friday evening when I started to get the screen flickering issue - random bars flashing across the screen and even complete blackouts for half a second at a time.
Undeterred, I followed some advice on this forum to use an SMC controller to increase fan speeds. After doing this, the machine has been perfect. This is my preferred solution to waiting another three weeks for another replacement which may well have the same problem.
Although the machine is in a very cold room and the temperature sensors report that the iMac is running very cool itself, it seems that some screen related component is not being cooled properly. Perhaps this is a build issue like with the old MacBook Pros and the thermal paste problems?
Anyway, I finally have my iMac i7. It is very fast, very quiet and beautiful to look at - now that it works.
For anyone interested, here's the times taken: ordered to UK address at UK online store on 24/10, arrived DOA on the 14/11, RMA on the 15/11, arrived at Apple 19/11, replacement PFS 30/11, shipped 1/12, arrived 4/12.