Paul, I booked my visit directly with the factory. Details are on their website. Mention, at the time of booking, that you vehicle is being built. The factory will look up the chassis number and give it to the tour guide who will find the chassis for you whilst you are on the visit.
Make sure it is being built when you are there to avoid disappointment.
Take loads of pictures and if appropriate ask the guys to sign your vehicle.
And enjoy the day.
