Just as a point of interest, Measham is actually in North West Leicestershire. It'd be a bit unfortunate to make a basic error such as the wrong County when trying to formulate a defence, if any.
I note that it is an average speed camera set up so the serial number of the system most probably is different to any serial number allocated to individual cameras. Is it possible that the NIP refers to the system serial number whilst the calibration certificates relate to the individual cameras, or even the other way round?
Cameras have been there for at least 7 years from what I can find.