Actually I read somewhere after googling 550d Long Exposure Noise Reduction following your answer that the noise reduction works best on lower ISO's and that with higher ISO's it can actually make it worse.
Here in the middle east if it gets below 20 deg its considered a bit chilly, its been down as low as 15 recently and everyone has been complaining bitterly!! Temperature induced noise and whether there is anything I can do about it is something I'd like to work out.
Here's the link re higher ISO's and increased noise with LENR http://www.imaging-resource.com/PRODS/T2I/T2IA5.HTM
Long Exposure noise reduction can be performed for exposures one second or longer, and works by taking a second "dark frame" of equal duration with the shutter closed, and then subtracting it from the first frame. This reduces or eliminates most noise generated by the sensor during long exposures at low ISOs, but can make noise worse at higher ISOs (at ISO 1,600 and above)