Hi Mandy, We currently have 15 people on the start line and expect to have 35 by the time entries close in June. There is a closed facebook group "Trans Britain Ultra" which you should ask to join. This is a group we set up so that people who are training for the 2012 race can ask questions of each other and previous participants.
Since we organise both events I can answer you anyway. There are two events in Thetford a sprint Duathlon which follows a 5K run, 20K cycle, 5K run format. For this the bike is compulsory.
There is also a 10K trail race on the same day which follows 2 laps of the run course. When we visited the course 3 weeks ago the paths were suitable for both road and trail shoes as the soil drains pretty well. It is impossible to say until the day how recent weather has affected the course.
There is a train which arrives at 08:14 from Marylebone. Last year this had 30-40 runners on it. If this train is delayed we will hold the start for a few minutes to allow everyone a chance to register.