There's a few police officers round here that could answer that, but I suspect it might not help you much. I could well be miles off here, but I think that the witnessing bit is for when they don't have a speedgun and need one copper to confirm the opinion of another about a vehicle's speed. You really want to go and visit the forum at
www.pepipoo.com and ask there. Even if this turns out not to be something you can use there may be other mistakes. If time is running short the Association of British Drivers have a list of helpful lawyers and a legal advice line
www.abd.org.uk