Can someone help with the way this spell works.
We can try. :)
What happens once the current target is Unconscious, eg on negative hit points? As the spell affects living targets it will stay on a given target until it is killed or the save is made, so do I need to get the fighters to finish of the current target, so the spell moves on and doesn't just stay the current downed target?
Sadly, yes, it does seem so.
Also if a target is stunned by this spell and then in its turn makes its save, the spell moves on to the next target (who now saves and could be stunned); So is the first target still stunned this round? As now the spell has moved off to someone else its not on this target any more?
Note that the target is dazed, not stunned. But, no, the spell seems to indicate that the save occurs as the turn starts, meaning that a successful save means that turn would not be lost.
Can the spell keep bouncing round loads of new valid targets until one of them saves immediately?
Do you mean fails? It can keep bouncing until a target fails, yes. If the target saves, then it keeps bouncing, provided a new target within range exists.