Your chances of winning a case are slim - Anton is correct.
The following might look promising, but the magistrates assume the camera never lies, and it is nigh on impossible to prove to them otherwise.
http://scienceandresearch.homeoffice.gov.uk/hosdb/publications/road-policing-publications/18-98-Guide-to-Calibratio1.pdf?view=Binary
Quote from the Home Office:
It is the purpose of the annual calibration check to provide reassurance to a court that a particular
device has been adequately maintained and continues to operate accurately.
http://www.homeoffice.gov.uk/about-us/freedom-of-information/released-information/foi-archive-research1/5687-calibration-enforcement?view=Html
Home Office research and science
Calibration Requirements for Traffic Enforcement Equipment
Date: Tue Feb 13 00:00:00 GMT 2007
The National Physical Laboratory is the UK's national standards laboratory. They maintain our national standards for a wide range of measurements including distance and time. They provide a specialist calibration service with the highest available accuracy which is used to calibrate test equipment against the appropriate national standard. You can read about these on their website
http://www.npl.co.uk.
The United Kingdom Accreditation Service (UKAS) is the national accreditation body that assesses organisations that provide certification, testing, inspection and calibration services. We expect test instruments used by manufacturers for the calibration of type approved traffic enforcement devices to be themselves calibrated by a UKAS accredited body which will ensure its accuracy can be traced to the national standard. You should refer to the UKAS website
http://www.ukas.com for more information.
The National Physics Laboratory
do not accept that the calibration certificates provided for cameras are to an acceptable standard. They have written to the Home Office as long ago as 1992 to make this point but have been ignored.
Try the web address, and see if you can get somebody to dig out that letter, if you want to take a chance.
I dont suppose the alleged offence was in Cumbria by any chance?
Email me if you want more information for future use - my addy is in my profile.