Update: Just went to the dealer - we checked their demonstrator bike which was delivered the same say as mine. The head bearings were way loose!
I got home and checked my bike, they are loose too! - this explains the rattling perfectly.
My bike is booked in for it's first service on Thursday and they'll be adjusting them then.
Can't wait to feel it when they are adjusted correctly.
Shame on Honda for sending the bikes out with mal-adjusted head bearings!