Hi Dean
The noise doesn't go away with the clutch depressed, which voids most of my theories too. Perhaps if the friction plate is down to the metal? I've even been wondering if it might be the speedo cable or the propshaft anti-vibration springs (probably not their proper name) but can't be sure.
As such I've handed it over to a professional (trustworthy local independent garage that specialises in older cars) to diagnose.
Good to get this out the way now as I plan to drive to Austria and back in October.