Further to the above, I've used a manual process to import a csv file of product codes and GTIN which might help anyone else.
The background to this is that the Google products feed requires the product data to have a GTIN otherwise products are disallowed ( this rule is being applied to an increasing range of products ).
In Hikashop I created a custom field in the products table `cust_product_gtin`
Then using PHPMyAdmin manually created a temporary table `tmp_product_gtin` with columns `product_code` and `product_cust_gtin`
Used PHPMyAdmin to import the csv to the temporary table
Then ran this query to update the Hikashop products table from the data in the temporary table ( replace "ap" with the prefix to your own database )
UPDATE `ap_hikashop_product` p
JOIN `tmp_product_gtin` t ON p.`product_code` = t.`product_code`
SET p.`product_cust_gtin` = t.`product_cust_gtin`
I now have 627 products with GTIN set instead of the 82 I got after trying to use mass action to import the data.