Do not sign anything yet! I think you might actually have a case here. If your car is registered on the V5 document as a March and NOT a Micra, then a mistake has been made and the evidence may not stand up in a court of law.
People have been let off speeding tickets for the incorrect vehicle name/time/date/registration details on the speeding ticket itself before.
Even the slightest error and the evidence can fall down. Remember that speed cameras rely upon people just accepting guilt without even questioning the details of the offence that they have been accused of.....
I'd recommend you try putting a question up on here
http://www.safespeed.org.uk/forum/index.php and you may find an answer to this.....