Since the car is here and it was bugging me, I just went out and pulled the dash top off again, disconnected the passenger side airbag and reconnected - listened for the audible 'click' of the connector - and refitted the dash top.
I also fitted the under glove box cubby I picked up last week.
A bit more googling also confirms that VS wisdom is sometimes like TB wisdom. This link from Volvoforums in the UK is probably more useful and insightful.
http://www.volvoforums.org.uk/showthread.php?t=130476&page=2
So I'll play with this solution first and then run it over to George if I cant clear the light.