What does your solicitor say ?
Have you visited PePiPoo to see what they have to say ?
Can you contest the validity of the alleged speed.
If so there is the Newton defence, see PePiPoo.
If you were not travelling the alleged speed, you could ask for the speed camera video, if there is one, under the Data Protection Act. The video can the be analysed to see if it can prove you were not travelling at the alleged speed, or possibly for some technical reason why the evidence is not admissible.
As you will be going to court anyway, you would also be able to get any video evidence under the disclosure rules, again see PePiPoo.
Also see the previous case;
http://www.safespeed.org.uk/forum/viewtopic.php?t=7014