I managed to get it done! (Eventually falling over this code snippet http://our.umbraco.org/wiki/reference/code-snippets/razor-snippets/render-razor-scripts-for-emails-and-more)

Hope this can help someone else in the same situation...

using umbraco; 

using umbraco.MacroEngines; 

using umbraco.cms.businesslogic.macro; 

      public static string GetOffer(int nodeId)

      {

        var macroEngine = new RazorMacroEngine();

        MacroModel model = new MacroModel();

        model.ScriptCode = "";

        model.ScriptLanguage = "cshtml";

        model.ScriptName = "OfferList.cshtml";

        return macroEngine.Execute(model, new umbraco.NodeFactory.Node(nodeId));

      }