Done!!! Ilvl496, first warlock and only dinged 90 3 weeks ago, took around 4 hours to complete, I could have got the killer soon looking back. I was so so focused on the mechanics of the fight that I was hardly DPSing Ken and as a consequence I was wiping to the imp waves at the end as it was taking to long. Tonight went back and focused purely on maximising damage to Ken following the advice earlier in the thread to really use the ember generation in the imp phase to nuke him.
Thanks to everyone in the thread for all of the tips and pointers, greatly appreciated. I really really hope Blizzard do not touch this great encounter, it is so well done.