Thanks Fengelz, I found I had to do this:

@Html.Raw(umbraco.library.RenderMacroContent(Model.secondaryBodyText.ToString(), Model.Id))

Yeah. 

Usually a good idea to stringify properties in razor. You never know what you'll be getting... :)

so I need some help with this. I can get the helper to render something, but it's not rendering the media item's umbracoFile property :-\... this macro works everywhere else on the site except through this helper call. It appears as if my macro params are not being populated correctly. Any Idea what is causing this?

@umbraco.library.RenderMacroContent(node.BodyText.ToString(), (int)node.Id)

The macro is configured like this:

  <?UMBRACO_MACRO showcaption="1" mediaitem="65663"
largeitem="65382" macroAlias="InsertImageItem"
enlargeimage="1" /> 

The macro looks like this in the code-behind of the ascx:

public partial class WRO_InsertImage : System.Web.UI.UserControl
    {
        public int mediaItem { get; set; }
		// snip other params

        protected void Page_Load(object sender, EventArgs e)
        {
            // try to get media item
            Media m = new Media(mediaItem);
            if (m == null)
                return;
				
			// snip the rest for clarity
        }
    }