In game menu

Hi all, im trying to make an in game menu to select levels.

I have a drag camera script to move left or right through the menu and each level sprite has a collider attached for when it is clicked. This all works fine if you click on the scene and drag the menu and then click the level you want. However if I want to drag the menu but accidentally click a level it registers the click before I drag the menu.

Does anyone have any idea of how I can stop it automatically clicking on the menu item when I want to drag the whole menu?.

Any help appreciated.

This is the drag script

 using UnityEngine;
 using System.Collections;
 
 // Attach to orthographic camera with rotation (0,0,0)
 public class DragCamera1 : MonoBehaviour
 {
 
     private Vector3 startMousePos;
     private float camerapositionx;
 
     void Update()
     {
         if (Input.GetMouseButtonDown(0))
         {
             PlayerPrefs.SetInt("MouseDragged", 1);
             startMousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             startMousePos.z = 0.0f;
             startMousePos.y = 0.0f;
         }
 
         if (Input.GetMouseButton(0))
         {
             Vector3 nowMousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             nowMousePos.z = 0.0f;
             nowMousePos.y = 0.0f;
             transform.position += startMousePos - nowMousePos;
             camerapositionx = transform.position.x;
             if (camerapositionx < 0.0f)
             {
                 transform.position = new Vector3(0.0f,0.0f,-12.0f);
             }
         }
 
         if (Input.GetMouseButtonUp(0))
         {
             PlayerPrefs.SetInt("MouseDragged", 0);
             Debug.Log("mouse up");
         }
     }
 }

and this is the objects click script

 #pragma strict
 
 var activevehicle1 : GameObject;
 
 function Start ()
 {
 }
 
 function Update () {
     
 }
 
 function OnMouseDown ()
 {
     if (Input.GetKey ("mouse 0"))
     {
         if(PlayerPrefs.GetInt("MouseDragged")==0)
         {
           PlayerPrefs.SetInt("MakeActiveLevel",1);
           activevehicle1.SetActive(false);
         }
     }
 }