Ignorance *of the law* is no defence but it's certainly a defence when giving evidence, if you don't know the answer to a question then the only answer you can give without perjuring yourself is "I don't know".
So if you were asked who was driving the car down XYZ High St. on a Saturday several months ago you would know? I certainly wouldn't, I don't keep a diary so I wouldn't be able to look in that. I work very regular hours and drive to work most days at the same time etc. but, every so often, my wife may take me or I may go somewhere else. Placing when these exceptions were several months later would simply be impossible.