Dave
I'm not sure with this, my advice is to get yourself onto
http://www.pepipoo.com
and join the members section, I think it'll cost you 30 quid, once in the members section post the details, you tend to get more help there.
The 100m calibration is probably just the distance that the law requires them to use for calibration, but the normal laser guns are regularly used at 500m and more. I don 't think that the calibration cert saying 100m will help you in this case, but check on pepipoo.
If you really think that you were slower that the alleged speed when you were inside the 40 limit then consider challenging it, but it could get costly if you are found guilty.
If the prosecution want to use the video as evidence in court then people are correct to say that this must be released to you 7 days before the court hearing, but to expect to have the case dropped because this proceedure was not followed is a bit of a longshot, most common would be for the court hearing to be re-scheduled and the court to instruct the prosecution to send you the video. The video will definately not be released to the defence unless it goes to court, however you maybe able to view the video for "identification purposes", which may allow you to see how fast you were going in the 40 limit, this would be at a specially agreed viewing, you maybe able to take an expert with you for this, for this, again you should ask pepipoo.
Give me a call if you like (07711 208994), no guarantees, but I can see if I can help, probably just be able to fill you in with a few points.
However wait until the last possible moment before you send back the NIP, right now there are several "right to Silence" cases in the ECHR being judged upon, a verdict is expected soon, however if the driver can be identified from the video, this will not help you.
Giles