Same system by the sound of it - I did find though that by sending machine to sleep - nitre/shift/eject and then pressing a key the display returned.
The machine was in fact still working even tho the screen had gone black - a second monitor continued to show 'live' images no problem
.
I did in fact take the machine to a nearby Apple Centre for a Genius Bar appointment. Needless to say the issue would not repeat itself whilst I was there but it was checked for video card and passed and the tech chap affirmed that as I was getting a picture/image on the second attached screen that the graphics card must be working ok.
It was agreed I would leave the machine and they would test it further-all very helpful.
They contacted me after 48 hours to say that whilst they had been able to replicate the problem of the iMac screen going black randomly there were no hardware faults causing it. It was suggested then that perhaps a program was a root cause- I did have a temp and fan speed monitoring program that the tech people mentioned had been noted to cause issues previously on occasion.
It was suggested and I agreed that the HD be wiped, a new OS Yosemite be installed and they monitor the impact of doing that.
They contacted me after a further 48 hours to say that no further occurrence of the screen going black had happened and the machine could be collected.
I did, re-installed data and apps from TM backup without a hitch(though was decidedly anxious as first time I had done such a thing). Machine behaved wonderfully until yesterday afternoon when the issue began again. There have been no new downloads of any software that I am aware of so I am at a loss again!
Hey ho - back to Apple I guess though trekking over to MK is a pain!
Thanks for the continued advice/support folks.