mrk
When they change the speed limit of the road, there has to be an Order made & approved by the Local Council. So the time frame that applies to all changes is precisely recorded and noted.
Therefor the dates and changes will be verifiable. However, notify the local Council, Police and Camera Partnership, (if you have one) immediately, of this error. The local Council has a legal obligation to the Government to apply the rules properly.
If you wish to notify me directly of this location I can alert the Local authorities of a potential error. It would be good to make others aware of this too.
If the error is proven then they should never send out any NIP's under the posted limit.
If signs have been removed then ensure that you have proof - a National Newspaper photographed at the location with the speed limits obvious will provide some evidence. Retain the newspaper as proof too of course.
A worrying experience.
(I am not a Lawyer and I cannot advise you - it is always sensible to seek proper legal advice.)