Thanks. However, I also read that the DS1307 has an alarm function. Would it be possible for that alarm function to trigger a thyristor to start the LED flashing? The thyristor could maybe be reset using a manual push switch. The DS1307 has a square wave output which I gather from a forum thread is not permanently switched on so maybe it is or could be related to the alarm function.
If that still isn't possible, have you any idea what is the cheapest microcontroller I should use?