If you see the attached image, you'll notice that the fan on the heatsink is facing towards the back of the case. (the fans on the heatsink are pre-attached). I think this is the wrong way round, but not sure. Any help much appreciated!
Before you go swapping it make sure that it isn't pulling air towards the back already, the fan may be set to pull to the back instead of push to the middle.
(Most) fans on upright heatsinks are pushing air into the heatsink. If that is so on yours. It is backward. If so the fan should be toward the front of the case with the rear case fan helping pull the hot air out.