Well I wasn't getting my hopes up when I RMA'd the CPU - but the replacement arrived yesterday, popped it in, and voila! Everything is back to normal.
The original appears to be dead as a dodo. Surprised that it died after 3 days - maybe it's a sign I should have gone with AMD over Intel?!
Anyhow - one further question - now I have the Aorus rather than the MSI. Any major advantage to updating the BIOS (it's from December) - I'm erring on the side of "If it ain't broke..." at the moment now that I have a PC back...! I've re-installed Windows (again) as I though that was probably sensible after changing mobo manufacturer. Took a 30min call with Microsoft tech support to re-activate Windows - I think I must have hit the re-activation limit. Ended up with a new product key.