Firstly, these sorts of threads are far better posted over at
http://www.pepipoo.com in their fightback forums.
Since you're here though:
I have been issued with a Notice of Intended Prosecution for a speeding offence (39mph in a 30mph) at Welwyn Garden City on the 8th January this year, which is dated 7th February.
This does invalidate the NIP (but not the S172 request, meaning you MUST respond, even if the NIP itself is out of time), but only if you are the registered keeper of the vehicle. If it's a hire car, company car, or if you've moved home or only recently bought the vehicle, then the police obviously have additional time to find the true keeper. If any of these are the case, then the 14-day limit is irrelevant.
1) Do I get to see the evidence against me before the case itself?
Yes. If you elect to have a hearing in court, you are entitled to full disclosure of all of the relevant evidence. Whether you receive it or not is a different matter, since the partnerships are
incredibly reluctant to give out entire tapes from the LTi20/20. If this happens though, it would only help your case, not hinder it.
2) Having seen the evidence am I then able if so inclined to plead guilty by letter?
Yes. You may change your plea at any time before the hearing, including by post. If your speed was significantly over the limit, and the magistrates are considering a ban though, you cannot have the case heard in your absence, and you will need to attend.