For Matt: there's no indication of Andy's tuning in the book I mentioned. I assume Andy has his top string tuned down a tone as he often does (so a mandolin tuned in this way would be GDAD). As I say, only an assumption - it's a long time since I heard the song. (And the Cm9 with B flat in the bass might have that inversion played on the guitar ... or at least the B flat.)
As for the Boyzone bit about 'Arthur', the first time I heard it, back in the 70s I did think 'Burt Bacharach' - but then BB wrote some classics, and PB has always been plugged into pop.