Air in my cold water system?

Hi, the radiator in our bathroom (only one in the house without a thermostat, and also the "highest" as it is a vertical towel rail type radiator) frequently needs bleeding as it gets air in it. I've recently noticed that when I turn in any cold tap in the house, there is a brief pause before water comes out - almost as though air is coming out first. I guess there must be a leak somewhere in my cold water system, but how on earth do I find out where the leak is? Is there something magic (Do I need Harry Potter) that I can add to the cold water tank that will fix the leak??