Many thanks Kuroi that was just what I was after ! Just done a test and seems OK. products_id was the index of course. This was what I used on the copies of the tables

UPDATE zen_products1 AS p, zen_products_extra_stuff1 AS et
SET p.products_ref = et.products_ref
WHERE p.products_id = et.products_id;
I've eyeballed the result and it looks fine but now I'm going to run do some more checks to makes sure it really is OK before doing the live run.

Thanks again.