So as a temporary measure I decided to try adding a new property to the Product document type. The prop is called bulkOffer1 and is basically another Merch Product Selectory.

Then on the Product page itself, if the umbraco product has a bulkOffer property then I pass that to the RenderAddToBasket:

 if (BulkOffers.Any())
            {
                foreach (var Offer in BulkOffers)
                {
                    if (Offer != null)
                    {
                        var NewModel = new ProductModelV2(Model);                            
                        NewModel.ProductData = Offer;
                        @Html.Action("RenderAddToBasket", "BasketOperations", new { area = "Bazaar", model = NewModel })
                    }
                }
            }

Note I've temporarily created a v2 of the ProductModel as the original does not contain a setter for ProductData. I now set it to the bulkOffer merchello product

public class ProductModelV2 : ProductModel
{
    public ProductModelV2(IPublishedContent content) : base(content)
    {
    }

    public new ProductDisplay ProductData { get; set; }
}

By the time this reaches the basket, in the form of a AddItemModel, the product details all directly related to the main/parent merch product.

[ChildActionOnly]
public ActionResult RenderAddToBasket(ProductModel model)
{
  AddItemModel addItemModel = new AddItemModel()
  {
    Product = model.ProductData,
    ContentId = model.Id,
    BasketPageId = model.BasketPage.Id,
    ShowWishList = model.ShowWishList,
    WishListPageId = model.WishListPage.Id,
    Currency = model.Currency
  };
  return (ActionResult) this.PartialView(ModelExtensions.ThemePartialViewPath((IMasterModel) model, "AddToCart"), (object) addItemModel);
}

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