In theory this is quite simple, however here is what I am doing

• Taken the code from the standard umbraco login code behind
• Override the validate user method to do my single sign on stuff
• Call the exactly the same code as if the user had clicked the login click button passing the username and password from the OnLoad()

If anyone has any ideas, am I missing setting a cookie somewhere etc. then please shout, the reason I am so confused is that I am using the exact same code as Umbraco does, the only real difference is that I am calling the exact same code that the OnClick() event calls in the OnLoad() e,g, I've done this:

 private void LogUserIn(string userName, string userPassword)
        {
            if (Membership.Providers[UmbracoSettings.DefaultBackofficeProvider].ValidateUser(userName, userPassword))
            {
                if (Membership.Providers[UmbracoSettings.DefaultBackofficeProvider] is ActiveDirectoryMembershipProvider)
                {
                    this.ActiveDirectoryMapping(userName, Membership.Providers[UmbracoSettings.DefaultBackofficeProvider].GetUser(userName, false).Email);
                }
                User u = new User(userName);
                BasePage.doLogin(u);
                if (u.DefaultToLiveEditing)
                {
                    int startNodeId = u.StartNodeId;
                    if (startNodeId == -1)
                    {
                        if (CMSNode.CountLeafNodes(-1, Document._objectType) <= 0)
                        {
                            throw new Exception("There's currently no content to edit. Please contact your system administrator");
                        }
                        startNodeId = new Document(CMSNode.TopMostNodeIds(Document._objectType).First<Guid>()).Id;
                    }
                    string url = string.Format("{0}/canvas.aspx?redir=/{1}.aspx", SystemDirectories.Umbraco, startNodeId);
                    base.Response.Redirect(url, true);
                }
                if (this.hf_height.Value != "undefined")
                {
                    this.Session["windowHeight"] = this.hf_height.Value;
                    this.Session["windowWidth"] = this.hf_width.Value;
                }
                if (string.IsNullOrEmpty(base.Request["redir"]))
                {
                    base.Response.Redirect("umbraco.aspx");
                }
                else
                {
                    base.Response.Redirect(base.Request["redir"]);
                }
            }
            else
            {
                this.loginError.Visible = true;
            }
        }

  protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            this.ClientLoader.DataBind();
            if (Membership.Providers["MyMembershipProvider"].ValidateUser("", ""))
            {
                var userFound = false;
                var userId = -1;
                var password = string.Empty;
                //todo there should be no need to do this but it is temp measure as a test
                if (Session["TempSessionUserId"] != null && Session["TempSessionPassword"] != null)
                {
                    userFound = Int32.TryParse(Session["TempSessionUserId"].ToString(), out userId);
                    password = Session["TempSessionPassword"].ToString();
                    if (userFound)
                    {
                        User u = new User(userId);
                        LogUserIn(u.LoginName, password);
                    }
                }

Thanks in advance for any help, it's driving me insane.