Gatso - 6 months later

[Matt S]

I received a letter from Thames Valley Police saying I've been done by a Gatso. They took their time thinking about it as the 'offence' occurred last October. Is there a time limit to which I need to be informed (e.g 2 weeks, 4 weeks etc) Standard stuff.... they're asking for the name of the driver. Is the answer 'How the hell do I know who was driving 6 months ago' valid in court, should it come to that?