By "speeding penalty notice" do you mean a Notice of Intention to Prosecute?
A NIP should be issued to the registered keeper within 14 days of the (alleged) offence I believe, though if she was stopped by the police rather than caught on camera then this won't apply – they will have given her a verbal NIP at the scene.
Note that if it's taken a while to track down the driver, this is outside of the 14 days. Ie, if the keeper and driver are different, the RK can't sit on it for a month before passing it on to the driver who then goes "14 days, hah!"
As for the "should have known better" argument; the real problem to my mind isn't the speeding in and of itself, but speeding whilst not paying attention to what she's doing. If you're going to drive fast you need to be constantly aware of what's going on around you, not seeing four speed traps would point towards observation issues.
If she needs her car for work and would be sacked if she lost her licence, you may have a case for claiming unnecessary hardship. Expect a hefty fine and perhaps a speed awareness course (which should really be coupled with a 'look where you're bloody going' course, when I'm in charge etc etc)