Lerp not working with UI button

Hi All,

I want to call this lerp function with a UI button. However, when I change it from an update to a public void, the object just moves instantaneously without the smooth lerp I'm after. Is there a way to leave it in update and use it as a function for a UI button?

Many Thanks

 {
     public Transform startMarker;
     public Transform endMarker;
     public float speed = 1.0f;
     private float startTime;
     private float journeyLength;
 
 
     void Start ()
     {
         startTime = Time.time;
         journeyLength = Vector3.Distance (startMarker.position, endMarker.position);
     }
 
     //void Update()
     public void moveLeft()
     {
         float distcovered = (Time.time - startTime) * speed;
         float fracJourney = distcovered / journeyLength;
         transform.position = Vector3.Lerp (startMarker.position, endMarker.position, fracJourney);
     }
 }