HP 7680 Duplex Problem [ Edited ]

[bcsom]

I have an HP7680 All in One with Duplex Unit which has been fantastic - until today!  

When printing 2-sided, it normally prints page 1, and then before spitting it out onto the tray, sucks it back in to the duplex unit, turns the paper round, and then prints on thr other side.  But today, it's spitting the paper out first goi, and then whirring away inside, telling me there's no paper inserted. I click ok., then it picks up a new piece of paper, and prints on that - reversed, as if it was printing on the back of page 1.  

Clearly somethings happenedm but I can't think what.  No new updates to my knowledge.  I've done the usual switch all off & restart; remove the unit & clean etc,m but no joy.  Please help,  as it's mightily inconvenient not being able to print bioth sides automatically, and has slowed printing down to an unacceptable level.  

I'm running Windows 7.

Many many thanks in anticipation...