First off, you should be looking at all of the potential reasons. The most common reason I have found in the Crystal Palace area is the lack of insulation in some of the homes. Your windows, too, could be causing a bit of a problem. I would recommend hiring someone who can take a look at the windows for you; there are many companies providing double glazing in Crystal Palace who could do the job for you.
You should not ignore this, but unexplained temperature dips are often the result of poor structural conditioning. It could result in lots of energy escaping your home, leading you to spend needless money just to heat your home. I think the first thing to look out for are gaps in and around your windows.