More Than Words by Extreme (sickly, I know, but it'd work)
To Be With You by Mr Big.
Maybe you could find an arrangement of (retch) Angels by Robbie Williams... although I think it gets played more at funerals than weddings, I don't know what the hell the lyrics are about.
Even worse... [cough] You're Beautiful by James Blunt [/cough].
I don't think Stairway is going to work - too long for a start.
Do you have to sing as well, Dave, or does it need to work as an instrumental?