Some original swannecks also have a 4mm hole at the lowest point just prior to the torpedo. Water is usually more of a problem in the tailpipe as this is the last part of the exhaust to get hot and so evaporate the trapped water. This is also why the back box is almost always the first part of the exhaust you have to replace.

Matt,
I've seen rear pipes with a drain hole on the 'U' bend, but never seen one on a swan neck pipe.
On the other hand, I drill a small hole in the underside of the torpedo, near the front where condensate will collect.
A self tapping screw goes in there at MOT time.

ken