Ok Peeps - I have everything working perfectly - see this map for it in action (or at least this pictures of it in the screen shots):
Here's how it was done in case anyone else needs to do it:
as per Ficool2
- light_dynamic - tick the No Light Models and No Light World in flags
Name - cart_tracker_main_01
< this way you can sequence them in your map so the arrows don't get confused
Entity to point at - your payload prop <as per Coding Ethan
Next - you can use a prop but I used a func_brush to get around the verticle changes in a my map - just make sure you position your arrow so that when the cart moves you can still see it and not have it disappear in the ceiling/floor/wall. so in mu case:
Name - Cart_follower_main_01
Parent - cart_tracker_main_01
I aligned the straight end of the arrow to the light < so it would act as the pivot point.
That's it - you should now have a working pointer.
My thanks again to Ficool2 and Coding Ethan < I had a hard time with this one!