I’m created a simplified activity using the ticker to show the time.
Here’s the results
The Teacher’s Dashboard shows both students with an elapsed time of 18
The student views show completely different times
So the behaviour on the teacher dashboard using ticker is different from when the activity was using timeSincePress but still inconsistent with what is shown on the student view 
Bit disappointed as I thought ticker was going to be the solution 
Back to the drawing board.