You seem to have good evidence (including the bungling efforts of the police attempting to get the date right). This is just a typo/technicality though yeah? Be wary of the possibility of the judge brushing a simple mistakes like that aside. I presume you were looking at your phone when noticed by the police, but not actually using it (call/text) at the time. Don't answer that one online (just incase).
If so then the most the police should be able to do you for is using a device whilst driving (this includes sat-navs, mp3 players, electric shavers,etc).
I think you should put forward all the evidence you have at the first available opportunity. If the judge looks at it in your favour then he can stop it going any further.
Hope it all goes well for you.
p.s. All of the above is totally my own opinion. I'm in no way a legal expert