Just to update, I have managed to do this now via javascript. I am building the HTML as follows...

@if (Model.ProductVariants.Any())
                        {
                            foreach (IProductVariantContent variant in Model.ProductVariants.Take(2))
                            {
                                int attCount = 0;
                                <tr data-nem="productitem">
                                    <td data-label="Product"><h4>@variant.Name</h4></td>
                                    <td data-label="Monthly cost"><strong class="price">@variant.Price</strong></td>
                                    <td data-label="Upfront cost"><strong class="price">£0.00</strong></td>
                                    <td data-label="Plan term">36 months</td>
                                    @foreach (ProductAttributeDisplay att in variant.Attributes)
                                    {
                                        <td class="hidden"><input type="hidden" class="option" name="OptionChoices[@attCount]" value="@att.Key" /></td>
                                        attCount++;
                                    }
                                    <td class="hidden">@Html.Hidden("ProductKey", variant.ProductKey)</td>
                                    <td class="hidden">@Html.Hidden("SuccessRedirectUrl", Model.Url)</td>
                                    <td class="hidden">@Html.AntiForgeryToken()</td>
                                    <td data-label="Quantity"><input type="text" id="quantity" title="Quantity Input" class="form-control" /></td>
                                </tr>
                            }
                        }
                        else
                        {
                            <tr data-nem="productitem">
                                <td data-label="Product"><h4>@Model.Name</h4></td>
                                <td data-label="Monthly cost"><strong class="price">@Model.Price</strong></td>
                                <td data-label="Upfront cost"><strong class="price">£0.00</strong></td>
                                <td data-label="Plan term">36 months</td>
                                <td class="hidden">@Html.Hidden("ProductKey", Model.Key)</td>
                                <td class="hidden">@Html.Hidden("SuccessRedirectUrl", Model.Url)</td>
                                <td class="hidden">@Html.AntiForgeryToken()</td>
                                <td data-label="Quantity"><input type="text" id="quantity" title="Quantity Input" class="form-control" /></td>
                            </tr>
                        }

I do the same for any related products.

Then have the following JS run on the click of the 'Add to Basket' button.

$('a#addProducts').click(function (event) {
    event.preventDefault();
    var products = $('tr[data-nem="productitem"]');
    var postData = "";
    var response = "";
    if (products.length > 0)
    {
        for (var i = 0; i < products.length; i++) {
            var product = products[i];
            var qty = $(product).find('#quantity');
            if (qty != undefined && qty.val() > 0)
            {
                postData = "Quantity=" + qty.val();

                var options = $(product).find('input.option');
                if (options.length > 0)
                {
                    $.each(options, function (index, option) {
                        postData += "&" + $(this).attr('name') + '=' + $(this).attr('value');
                    });
                }

                var key = $(product).find('input#ProductKey');
                if (key != undefined)
                {
                    postData += "&" + $(key).attr('name') + '=' + $(key).attr('value');
                }

                var url = $(product).find('input#SuccessRedirectUrl');
                if (url != undefined) {
                    postData += "&" + $(url).attr('name') + '=' + $(key).attr('value');
                }

                var token = $(product).find('input[name="__RequestVerificationToken"]');
                if (token != undefined) {
                    postData += "&" + $(token).attr('name') + '=' + $(token).attr('value');
                }

                if (MUI.Settings.Endpoints.basketSurface !== undefined && MUI.Settings.Endpoints.basketSurface !== '') {
                    MUI.AddItem.postUrl = MUI.Settings.Endpoints.basketSurface + 'AddBasketItem';
                }

                if (postData != '')
                {
                    $.ajax({
                        type: 'POST',
                        url: MUI.AddItem.postUrl,
                        data: postData
                    }).then(function (result) {
                        MUI.emit('AddItem.added', result);
                        MUI.Notify.info('Successfully added item to basket');
                    }, function (err) {
                        MUI.Logger.captureError(err);
                    });
                }
            }
            postData = "";
        }
    }
});

With this I can then go through all of the items (whether it is a variant of a product or not) on the page and find the quantity entered, the variants' options (if any), and add them incrementally to the basket.

Thanks, James