If anyone else needs help on this one I achieved it with the below:

    var contentService = ApplicationContext.Current.Services.ContentService;
    var content = contentService.GetById(1343);

    var data = new GridRowCollection(content.GetValue("orderItems").ToString()); //DataType Alias

    int gridRowCount = data.Count();

    GridCell gcProductName = new GridCell();
    gcProductName.Value = "Product Name";
    gcProductName.Alias = "productName";

    GridCell gcProductGategory = new GridCell();
    gcProductGategory.Value = "Category Name";
    gcProductGategory.Alias = "category";

    GridRow gr = new GridRow();
    gr.Add(gcProductName);
    gr.Add(gcProductGategory);

    data.Insert(gridRowCount, gr);

    content.SetValue("orderItems", data.ToString());

    var result = contentService.SaveAndPublish(content);