Okay, found it. For anybody else wanting to do this you change the search string as described by Mav666 above, then you look for the following string and make the same change there.
CODE
$select_str = "select distinct " . $select_column_list . " m.manufacturers_id, p.products_id, pd.products_name, pd.products_description, p.products_price, p.products_tax_class_id, IF(s.status, s.specials_new_products_price, NULL) as specials_new_products_price, IF(s.status, s.specials_new_products_price, p.products_price) as final_price ";
So if you are running the header tags contribution and want to include the keywords in the search, the field you need to add is pd.products_head_keywords_tag.
Ian