Regarding the steep slope, I don't know the exact reason that the hovercraft is launched from the truck, but I think it has something to do with were the hovercraft is stored when not in use. The slope it self does not appear to be very steep.
I will ask them the next time that I'm at the station.
The Hovercraft is kept on the lorry. Its the rig, so that it can be taken anywhere to launch and be recovered. They have large areas to cover and depending on the weather, tide etc, it might be quicker to take the Hovercraft nearer by road and launch it than to launch it from its station.