Solved

No doubt a silly question - but I don't know the answer

When you spectate a race, in the top right hand corner is the race leaderboard that is live - within the list of rasers there's a line that appears to shift, but I don't know what it means or represents.

Does anyone know?

The line you are likely seeing is a green color. It indicates the position one driver must finish above in order to receive the sponsor bonuses for the race. The green line is a convenient indicator of what the race objective is.

Although, I don't think it's supposed to be moving if that's what you saw.

The line moves all the time. It is supposed to represent where you are in relation to your race objective. But it doesn't do that. It wanders about according to its own weird logic. It's no big deal really. I'm sure the admins have more important tings to look at.

Doesn't it just move to show you that some drivers in front of you have done less pits than you, so you may end up on higher position than you are now, making your goal for a race lower on the grid?
For example, your goal is getting 5th, but 2nd and 3rd places haven't done 2nd pits yet, and you've done them and one of your drivers is 8th now, so 5th+2places, becouse 2 drivers will go to pit = you need to be 7th right now to end the race at 5th position. So green line will be between 7th and 8th, so you are just one place to getting the goal of the race.
This would make sense, if it isn't the case, then I can't think of any other reason.

He is spectating. He doesn't have a driver. That line shouldn't be seen while spectating.