I recommend you downloading and installing the latest driver version we have from our side. It can be downloaded at the following link:
Please choose the correct version according to the operating system
Intel® HD Graphics Driver for Windows* 7/8/8.1 64-bit
Intel® HD Graphics Driver for Windows* 7/8/8.1 32bit
It is important to mention that our drivers are generic driver and sometimes these cannot be installed on other systems or the installation of them could overwrite their customizations and possibly change the functionality of the graphics, but you can give it a try.
Oddly, that's what I'd already done - except it was updated previous driver win64_152822 rather than win64_152823 and the problem has disappeared. I'd also swapped Primary/secondary monitors over - so will swap them back in a few days time to see if the problem reappears.
It's odd tough that the problem should occur at all after all this time !
Incidentally, are there any settings for HDMI behaviour ? I've gone back to using VGA for reliability because the big issue with a break in HDMI link causes the computer to forget there's a 2nd display attached. This doesn't seem to apply with VGA - so a simple matter of plugging back in and the 2nd screen works without having to set it up again. Is there a way to replicate that using HDMI ?
HDMI cable connectors are total crap - hence the return to using VGA.