What's wrong with this picture? Give up? I'll tell you...
The Scalextrc Sport Digital set does not come with a start grid piece. In fact, all you get is a single white line on the digital power base half straight, that runs across a couple of the little holes for the lap counter sensors. But... you can't use that to line up your cars because...
...the set also comes with a sheet of paper, an 'Additonal Instruction' that states, "Place the race car no less than 10cm behind the white Finish Line on the digital power base so that all the semi-circular cut-outs in the track rails are ahead of the car. This must be done to ensure correct lap counting."