Hi I have a problem where my view data is not persisted. I've read the other threads but I think my problem might be a bit different.

Firstly I have a template page that renders the child nodes umbraco.library.RenderTemplate

foreach(var page in dn.Children)
{
    if(page.NodeTypeAlias!="Page")
    {
   @MvcHtmlString.Create( umbraco.library.RenderTemplate(page.Id));
    }
}

This in turn calls my form template which call my partial view with a form

@model FormViewModel
<div style="padding-top:0px" id="wrapper">
    <!-- PAGE TITLE -->
    <section id="contact" class="container">
     <div class="row">
            <!-- FORM -->
            <div class="col-md-8">
                <h2>Complete the form below <strong><em></em></strong></h2>
                @if (ViewData.ContainsKey("SuccessMessage"))
                {
                    <div class="alert alert-success">
                        <i class="fa fa-check-circle"></i>
                        <strong>@ViewData["SuccessMessage"]</strong>
                    </div>
                    return;
                }
                @if (ViewData.ContainsKey("ErrorMessage"))
                {
                    <div class="alert alert-error">
                        <i class="fa fa-check-circle"></i>
                        <strong>@ViewData["ErrorMessage"]</strong>
                    </div>

                }
                @using (Html.BeginUmbracoForm("SendEmail", "FormSurface",Model))
                {
                    <div class="row">
                        <div class="form-group">
                                <div class="col-md-6">
                                <label>Course Date *</label>
                                @Html.TextBoxFor(x => x.CourseDate, new { @type = "date", @class = "form-control", @name = "message", @id = "message" })
                            </div>
                        </div>
                    </div>
                    ...

And My Surface Controller

public class FormSurfaceController : Umbraco.Web.Mvc.SurfaceController
{
    [HttpPost]
    public ActionResult SendEmail(FormViewModel model)
    {



        //model not valid, do not save, but return current umbraco page
        if (!ModelState.IsValid)
        {
            //Perhaps you might want to add a custom message to the ViewBag
            //which will be available on the View when it renders (since we're not 
            //redirecting)      
            try
            {
                ViewData["ErrorMessage"] = "Form Error";
            }
            catch
            { }

            return CurrentUmbracoPage();
        }

ViewData["ErrorMessage"] is null when I return to CurrentUmbracoPage. Could this be because of my

@MvcHtmlString.Create( umbraco.library.RenderTemplate(page.Id));

in my temlate ?