LookAt script Issues

Hi there.

I'm currently trying to setup up a camera rail system and have stumbled into a bit of a problem. I have a path for my camera to move along, and a target for my camera - however because my camera is using a LookAt script, once the game starts playing I dont seem to have any way of controlling the local tilt (or 'roll' i should say) of the camera. I dont suppose anyone could help me alter my LookAt script so as to 'release' my camera's ability to tilt.

Ideally I would like to have a separate public float (called Tilt perhaps) which I can use to alter the local z-rotation of my camera.

Below is my script - Any help is greatly appreciated:

 using UnityEngine;
 using System.Collections;
 
 /*        
  *         This is a modified C# version of Unity's standard SmoothLookAt script.
  * 
  *         The main benefit of this script is that it allows the user to not rotate an object on all of its axis to look at a target.
  *         For example, you might want a gun turret to swivel on the spot to face a player, but to not rotate the mesh 'vertically' if the player jumps.
  *         As a result the most common configuration with this script is to make the X-axis and Z-axis true, but to leave Y-axis false.
  * 
  */
 
 
 public class LookAtAxisLock : MonoBehaviour
 {
     public bool Smooth = true;
     public float damping = 6.0f;
     
     public GameObject RotatingObject;
     public GameObject LookTarget;
     public bool X_axis = true;
     public bool Y_axis = true;
     public bool Z_axis = true;
     private float X_position;
     private float Y_position;
     private float Z_position;
     
     void Start ()
     {
         // Make the rigid body not change rotation
            if (RotatingObject.rigidbody)
         RotatingObject.rigidbody.freezeRotation = true;
     }
     
     void LateUpdate ()
     {
         
         //Update the lookTarget's current X,Y,Z locations
         if (X_axis)
         {
             X_position = LookTarget.transform.position.x;
         } else {
             X_position = RotatingObject.transform.position.x;
         }
         if (Y_axis)
         {
             Y_position = LookTarget.transform.position.y;
         } else {
             Y_position = RotatingObject.transform.position.y;
         }
         if (Z_axis)
         {
             Z_position = LookTarget.transform.position.z;
         } else {
             Z_position = RotatingObject.transform.position.z;
         }
         
         if (Smooth)
         {
             // Look at and dampen the rotation
             Vector3 relativePosition = (new Vector3(X_position, Y_position, Z_position) - RotatingObject.transform.position); // NB: For the Quaternion to work a vector must be created from the 'origin' (kindof)
             Quaternion rotation = Quaternion.LookRotation(relativePosition);
             RotatingObject.transform.rotation = Quaternion.Slerp(RotatingObject.transform.rotation, rotation, Time.deltaTime * damping);
         }
         else
         {
             // Just lookat
             RotatingObject.transform.LookAt(new Vector3(X_position, Y_position, Z_position));
         }