Hi there Daogroupie,
My daughter did a mock test in June and found it very useful. I work in Sutton so know the area fairly well. You remember the environment correctly! The school is perhaps 10-15 minutes away from the station (I am always cautious and an early bird so would add some!) I'm not so sure about the trains so it's probably best to use the journey planner on South West trains (or similar) website (that normally tells you exactly how long the journey will take with walking time too). The school is across a road called Throwley Way (right at Wilkinsons as you walk down the high street from the station). As you can tell I'm not the best at giving directions!
The weather was fine when we went and the hoards waited in the playground outside, then we lined up and my DD was registered at the door and we said our goodbyes. She took a pencil and rubber and I think that was all she needed - I'm pretty sure it was all multi-choice. I'm not certain but I think they had a short break after each and certainly left their desks for that. She did say that a good explanation was given before they started.
They then come out to all the anxious mummies and daddies back into the playground where we dropped up. It was very good practice and my daughter learnt several important lessons. Though the results do appear to be a bit random and unfathomable, she learnt the importance of timing and said herself she will be less anxious when it comes to the real thing. I hope your DD finds benefit from it too. Definately not one to stress over!!