Yosemite install won't boot into any mode

Really sorry if this has been covered before but I spent a good time searching and couldn't find any existing posts with symptoms to the same extent.
I've got a 2008 iMac 27" and have the latest version of Yosemite installed (10.10.1 I guess). I upgraded to Yosemite probably 2-3 weeks ago and everything has been fine. Then yesterday when I woke it up from sleep it effectively froze - no mouse or keyboard input. So I did a reset by holding down the power button, and when I turned it back on it went to the apple logo boot screen with a progress bar that gets to just below 50% and stuck there. I've restarted quite a few times and left it running up to six hours and still it doesn't boot.
Now this is similar to the issue here - OSX Yosemite won't boot after install on mid 2010 iMac - except I am unable to boot into any mode, and the only startup keys accepted are the P-RAM reset. It won't boot into recovery mode (command+R), safe mode (Shift), startup drive select mode (Option), diagnostics mode (D)... nothing
I've booked into a Genius Bar appointment when the Apple stores reopen after Christmas, but I'm hoping that is a last resort and to be honest I'm unsure what they can do for me except to say it's a lost cause.
Are there any suggestions on what could be happening and why it's ignoring the startup modes? I would have suggested the keyboard not being recognised except the P-RAM command works perfectly, so it's obviously seeing it. I'm a little scared that because it won't let me into a boot device menu (which must operate independent of any HDD) then there's an issue ahead of that in the boot sequence, so possibly the motherboard is shot...
Sorry if I seem a little desperate. I had a HDD crash two years ago to the day (great Christmas present Apple) and it kind of puts a dampener on things.
Happy holidays and I'm grateful for any help that anyone can offer

Hello there, semerton.
Great job on troubleshooting the issue on your own so far. The following Knowledge Base aritcle offers some additional steps to try for resolving your issue:
Mac OS X: Gray screen appears during startup - Apple Support
Thanks for reaching out to Apple Support Communities.
Cheers,
Pedro.

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