Definitely ask for the photo so you can identify who was driving given that the vehicle has multiple drivers ..... this will give you the evidence that they have ...... you can then make an informed decision from that point onwards. Also ask for the speed camera calibration certificate as they have been known to be out of date and therefore ineligible as evidence.
In many police force areas across the country, it is an agency that man the speed vans, and as is the same for speed cameras, the data is passed onto a reviewing police officer to decide upon which instances are sent forward for prosecution.
The legal limit for prosecution is speed limit x 1.1+1, ie 34 in a 30mph zone, 45 in a 40mph zone and 67 in a 60mph zone. I believe it is the same for motorways too, but there is a tendency to be a little more lenient and use 80mph otherwise the courts would be completely full.
There are numerous reasons why intended prosecution is withdrawn. Contesting the NIP dictates that the authorities take a second look at the evidence they have compared with your reason & any evidence you have for not pleading guilty. They then make an informed decision on whether to proceed or not. Many cases are dropped simply because the offender contests the NIP and it is a borderline speed - such as yours.
If the authorities decide to proceed, you get your opportunity to present your evidence and reasoning to the judge - you dont need to engage a solicitor but it would be much better to do some homework to identify previous cases where the use of tachographs have proved to be reliable evidence against the accusation. Have your tacho calibration dates and certs available (compare calibration of tacho requirements against calibration of speed camera requirements). Demonstrate your speed on a map showing other speed limits to prove you are basically a speed abiding driver. Details of your driving history may or may not prove beneficial too (depending upon your previous !!), ie 20 yrs without a ticket, miles driven per year etc etc
I am sure more knowledgable others will identify areas for consideration that I have omitted ............ good luck and keep us informed ........