I have a problem posting an SOP to api.columbus.sage.com/.../sop_orders

It comes down to tax_code_id.
We have products that are either Standard Rated ( VAT @ 20%) or Zero Rated (0%)

When I post an order with lines that have products that are Standard Rated - Sage will accept the order
When I post an order with lines that contain products that have Zero Rated - I get a 400 error.

So the JSON packet below contains 1 standard and 1 zero rated product.
This JSON packet fails to post.

If I remove the product "TEST" the order succeeds
If I specify the standard rate tax code ID within the line for TEST the order succeeds
If I remove all reference to tax codes on all lines, the order fails.
If I post 1 line with a Standard Rate product the order succeeds
If I post 1 line with a Zero Rated product the order fails

I dont think its a JSON issue, although happy to be proved wrong!

The tax_code_id is the tax_code_id I get from the product object.
The ID also validates when I query tax codes independantly.

Can anyone suggest what might be going wrong?

{
    "customer_id": 499999,
    "document_no": "WEBORDER-TEST",
    "is_editing": false,
    "customer_document_no": "WEB ORDER 23-01",
    "document_discount_percent": 0,
    "payment_with_order": true,
    "invoice_payment_with_order_immediately": true,
    "payment_value": 50.00,
    "payment_reference": "TEST_1AbYLOmzY",
    "payment_method_id": 530316,
    "delivery_address": {
        "postal_name": "Chris Testing",
        "address_1": "Some Road",
        "city": "Somecity",
        "county": "Acounty",
        "country": "GB",
        "postcode": "NE1 1NE",
        "contact": "Chris",
        "telephone_number": "07967123456",
        "email_address": "[email protected]"
    },
    "lines": [
        {
            "line_type": "EnumLineTypeStandard",
            "product_id": 937295,
            "warehouse_id": 931948,
            "code": "MYPRODUCT",
            "description": "MYPROD (Standard Rated)",
            "line_quantity": 1,
            "tax_code_id": 1760,
            "selling_unit_price": 30.00,
            "selling_unit_price_overridden": true
        },
        {
            "line_type": "EnumLineTypeStandard",
            "product_id": 938423,
            "warehouse_id": 931948,
            "code": "TEST",
            "description": "Test (Zero rated transactions)",
            "line_quantity": 2,
            "tax_code_id": 1767,
            "selling_unit_price": 4.00,
            "selling_unit_price_overridden": true
        },
        {
            "line_type": "EnumLineTypeCharge",
            "product_id": 1022221,
            "warehouse_id": 931948,
            "code": "CAR_1",
            "description": "Website Carriage",
            "line_quantity": 1,
            "selling_unit_price": 10.00
        }
    ]
}