How to pre-populate a BizForm control with existing data

Hi. I have a portal page template in a checkout process that hosts a BizForm control to collect some additional information for a user during checkout. When valid data is submitted on the form I store the submitted data in the cart item custom data for future processing.

However, if the user wants to add another item to their basket and re-enters checkout I want to retrieve the data already submitted previously and pre-populate the form fields with that data so the user doesn't have to fill it in again. I am struggling to get the data to show in the form fields so a bit stuck on what to do next. Here is my code:

            form.SubmitButton.Visible = false;
            form.ReloadData();

            var cartItem = CurrentCart.CartItems
                .FirstOrDefault(ci => ci.CartItemID.ToString().Equals(cartItemId));

            var existingFormDataId =
                ValidationHelper.GetInteger(cartItem?.CartItemCustomData.GetValue("FormItemID"), 0);

            BizFormItem currentData = null;

            if (existingFormDataId > 0)
            {
                currentData = BizFormItemProvider.GetItem(existingFormDataId, $"BizForm.{form.FormName}");
            }

            if (currentData != null)
            {
                foreach (var field in form.Fields)
                {
                    var fieldValue = currentData?.GetValue(field);
                    form.Data.SetValue(field, fieldValue);
                }

                // What next?
            }

Can anyone suggest what I am missing here and how I can get the existing data to pre-populate in the form fields? Many thanks.