Accoring to the ACPO guidelines (
www.acpo.police.uk), if the site is un-supervised, the central ticket office should have checked the 2-stage photographs and verified the speed before sending the NIP out. (They can do this using the white lines painted on the road).
I would suggest sticking out to gt a copy of the images. I have an outstanding speeding issue I have taken up with Northumbria Police. The police were, at first, very reluctant to provide the images, but having driven 40 miles to the ticket office, they oblige.
I've been told at a couple of stages they don't have to provide you with the information but I would contest this.
One excuse you can use (if they've sent the NIP and you haven't yet replied) is that you need the photo's to identify who was driving it as you're unsure. They'll probebly refuse. If you really want to get the goat up, use the data protection act. If they have your car (as long as you're the ownder) on film, you're entitled to the images! It's the law and is an issue I'm currently persuing with my local MP.
If you're convinced your speed was below what the Gatso said, hold out. It won't be an easy challenge, but you can't hold your hands up to a crime you haven't commited. Or they'll just keep on doing this!