Keyboard and mouse connected straight to the PC in question. There was/is nothing else inline .
The mouse has no balls - it's optical. It's a USB mouse with a USB to PS/2 converter plug.

Interestingly in failsafe mode I noticed a message:
PNP: No PS/2 controller found. Probing ports directly.

Still no keyboard action.

The scsi question in expert mode was the first I was presented with.

I shall steal a keyboard from someone else in a minute and try with that!