I sorted it eventually 
Just in case anyone else is interested. Please amend the paths to suit your situation if you are using overrides.
Also NOTE I am no expert. I just analyse and use trial and error so please use with caution.
in file \includes\modules\pages\account\header_php.php
Find
PHP Code:
$orders_query = "SELECT o.orders_id, o.date_purchased, o.delivery_name,
o.delivery_country, o.billing_name, o.billing_country,
ot.text as order_total, s.orders_status_name
add o.billing_company to the end to look like this
PHP Code:
$orders_query = "SELECT o.orders_id, o.date_purchased, o.delivery_name,
o.delivery_country, o.billing_name, o.billing_country,
ot.text as order_total, s.orders_status_name, o.billing_company
Then in the same file find
PHP Code:
$ordersArray[] = array('orders_id'=>$orders->fields['orders_id'],
'date_purchased'=>$orders->fields['date_purchased'],
'order_name'=>$order_name,
'order_country'=>$order_country,
'orders_status_name'=>$orders->fields['orders_status_name'],
'order_total'=>$orders->fields['order_total'],
);
Change to
PHP Code:
$ordersArray[] = array('orders_id'=>$orders->fields['orders_id'],
'date_purchased'=>$orders->fields['date_purchased'],
'order_name'=>$order_name,
'order_country'=>$order_country,
'orders_status_name'=>$orders->fields['orders_status_name'],
'order_total'=>$orders->fields['order_total'],
'order_billcompany'=>$orders->fields['billing_company']
);
In \includes\templates\template_default\templates\tpl_account_default.php
Find
PHP Code:
<td><address><?php echo zen_output_string_protected($orders['order_name']) . '<br />' . $orders['order_country']; ?></address></td>
Change it to
PHP Code:
<td><address><?php echo zen_output_string_protected($orders['order_name']) . '<br />' . $orders['order_billcompany']; ?></address></td>