Ok, update on this from the con. Originally the Watch was going to be created by a company called Prime Focus. This has now changed with the creation of a new production company called Narrativia. The company consists of Terry Pratchett, Rob Wilkins, Rod Brown (producer), Guy Burt (head writer) and Rhianna Pratchett (writer). The new company has obtained the rights back. They are at the very early stages of two projects: Good Omens and The Watch. The Watch series will take place some time after Snuff (though the line they drew in the sand appears to be moving) and will consist of 13 episodes per season - the guys are planning on at least five seasons. The budget for the first season is £26 million. The plan is to build part of Ankh-Morpork for season one, and then for each consecutive season add to it to expand the city giving it a more geographical feel. The Watch series will be cannon, if a member of the watch dies in the tv series Terry will not having them in future books etc. Of course being dead is not always a problem on Discworld.
The series will have an overall story ark and individual, for want of a better word, crimes of the week.
The team have three new characters so far and will use them to introduce us to the series. The new characters are new recruits to the Watch so we will learn with them. It will of course keep the existing Watch so expect to see Vimes, Carrot etc. One of the new characters is Igorina who is the forensics expert and the team have had great fun working out how forensics might work on Discworld.
A proper press release is due in a few weeks time. But this all sounds very exciting.