plastic tank?
if it's a steel tank it should be like S4 in post #3
http://ukmonster.co.uk/monster/showthread.php?t=60641
but I thought they'd fully switched over to plastic in 2004, maybe you have a late registered model
in either case the return pipe is the one that comes from the pressure regulator ('D' in the S4 diagram in the above link)
ps. you don’t have carbs, they’re ‘throttle bodies’ with fuel injection