Logout not working, .AspNet.ApplicationCookie is not being deleted

When I click on logout I am taken to util/logout.aspx however when I navigate back to the main page of our website I am still logged in and can access the CMS backend.

From what I can tell the cookie .AspNet.ApplicationCookie is not being deleted.

In our PageControllerBase.cs we have:

        public ActionResult Logout()
        {
            UISignInManager.Service.SignOut();
            return RedirectToAction("Index");
        }

Which when I compare it to the Alloy EpiServer demo appears to be identitical.

Where else do I need to look to check what could be the issue?

In case it is also relevant, we are using a custom login page:

namespace Project.Site.Controllers
{
    public class CustomLoginController : Controller
    {
        private UISignInManager uiSignInManager = ServiceLocator.Current.GetInstance<UISignInManager>();
        private UIUserProvider uiUserProvider = ServiceLocator.Current.GetInstance<UIUserProvider>();
        public ActionResult Index()
        {
            return View(Global.CustomLoginView);
        }
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        [ValidateInput(false)]
        public ActionResult LocalLogin(CustomLoginViewModel model)
        {
            if (ModelState.IsValid)
            {
                bool result = uiSignInManager.SignIn(uiUserProvider.Name, model.Username, model.Password);
                if (result)
                {
                     return Redirect(UrlResolver.Current.GetUrl(ContentReference.StartPage));
                }
            }
            ModelState.AddModelError("LoginError", "Login failed");
            return View(Global.CustomLoginView, model);
        }
    }
}