Custom login page with custom authentication

Can anyone point me in the right direction, I have a custom logon form and it authenticates a user correctly and seems to be working fine but won't redirect to the admin interface. The redirect goes to /Admin/CMSAdministration.aspx. Why if authenticated, won't it redirect to the admin user interface for that user?

Authenticate Method in LogonForm web part

if (MFAuthenticationHelper.IsMultiFactorRequiredForUser(userName))
{
    var plcPasscodeBox = Login1.FindControl("plcPasscodeBox");
    var plcLoginInputs = Login1.FindControl("plcLoginInputs");
    var txtPasscode = Login1.FindControl("txtPasscode") as CMSTextBox;

    if (txtPasscode == null)
    {
        return;
    }
    if (plcPasscodeBox == null)
    {
        return;
    }
    if (plcLoginInputs == null)
    {
        return;
    }

    // Handle passcode
    var passcode = txtPasscode.Text;
    txtPasscode.Text = String.Empty;

    var provider = new CMSMembershipProvider();

    // Validate username and password
    if (plcLoginInputs.Visible)
    {
        if (provider.MFValidateCredentials(userName, Login1.Password))
        {
            // Show passcode screen
            plcLoginInputs.Visible = false;
            plcPasscodeBox.Visible = true;
        }
    }
    // Validate passcode
    else
    {
        if (provider.MFValidatePasscode(userName, passcode))
        {
            e.Authenticated = true;
        }
    }
}
else
{
    var isValid = Membership.Provider.ValidateUser(userName, Login1.Password);
    e.Authenticated = isValid;
}

Logged in method in LogonForm web part

 // Ensure response cookie
CookieHelper.EnsureResponseCookie(FormsAuthentication.FormsCookieName);

// Set cookie expiration
if (Login1.RememberMeSet)
{
    CookieHelper.ChangeCookieExpiration(FormsAuthentication.FormsCookieName, DateTime.Now.AddYears(1), false);
}
else
{
    // Extend the expiration of the authentication cookie if required
    if (!AuthenticationHelper.UseSessionCookies && (HttpContext.Current != null) && (HttpContext.Current.Session != null))
    {
        CookieHelper.ChangeCookieExpiration(FormsAuthentication.FormsCookieName, DateTime.Now.AddMinutes(Session.Timeout), false);
    }
}

// Current username
var userName = Login1.UserName;

// Get info on the authenticated user
var ui = UserInfoProvider.GetUserInfoForSitePrefix(userName, SiteContext.CurrentSite);
var siteName = SiteContext.CurrentSiteName;

// For site prefix user, authenticate manually
if (ui != null)
{
    if (UserInfoProvider.UserNameSitePrefixEnabled(siteName) && UserInfoProvider.IsSitePrefixedUser(ui.UserName))
    {
        AuthenticationHelper.AuthenticateUser(ui.UserName, Login1.RememberMeSet);
    }
}
// Check whether safe user name is required and if so get safe username
else if (RequestHelper.IsMixedAuthentication() && UserInfoProvider.UseSafeUserName)
{
    userName = ValidationHelper.GetSafeUserName(userName, SiteContext.CurrentSiteName);

    if (UserInfoProvider.UserNameSitePrefixEnabled(siteName))
    {
        // Check for site prefix
        ui = UserInfoProvider.GetUserInfoForSitePrefix(userName, SiteContext.CurrentSite);
        if (ui != null)
        {
            userName = ui.UserName;
        }
    }

    AuthenticationHelper.AuthenticateUser(userName, Login1.RememberMeSet);
}

// Set culture
var drpCulture = (CMSDropDownList)Login1.FindControl("drpCulture");
if (drpCulture != null)
{
    var selectedCulture = drpCulture.SelectedValue;

    // Not the default culture
    if (selectedCulture != "")
    {
        // Update the user
        if (ui != null)
        {
            ui.PreferredUICultureCode = selectedCulture;
            UserInfoProvider.SetUserInfo(ui);
        }

        // Update current user
        MembershipContext.AuthenticatedUser.PreferredUICultureCode = selectedCulture;
    }
}

// Return url is not specified or is relative path or hash is valid
URLHelper.Redirect(DefaultTargetUrl);

Thank you, Alex