So... selecting the USB stick directly from the BIOS and not waiting for Chimera overcame the 'Mach_Kernel' challenge! I have Yosemite up and running now...
The really bad news is that it somehow did affect my Win7 installation which does not work anymore.... and all my son's gaming stuff and data are there and I did not know he had no backup. Big trouble.
Win7 is still there as an option in Chimera, but does not start and the Windows installation disk cannot repair as it does not even recognise the installation. Very surprising that the update (not new installation) to Yosemite did affect the Windows partition, clearly did not expect that as it always worked fine in the past.
ANYBODY ANY IDEAS WHAT TO DO???
Thanks, guys...