Thanks for the reply. 20k prior posts on a mature, fairly complex site, to which we now want to add a directory feature. The GD plugin, although I cannot see it well because the site does not really load with it activated, (though sometimes a page loads after over 1 minute), appears to add a directory feature to each post. Even loading the plugins page to deactivate GD is difficult while it is activated.
The server is good for the site without GD plugin, works OK with the ‘Business Directory’ plugin (which is not what we want), and was good for the old Drupal site including mapping from which the present WP site was migrated. There is nothing in my error log (apart from ‘mysql server not found’ when mysql dies).
You are welcome to login details. It may not be safe to publish FTP and Mysql credentials on a public forum. We have just spent many hours removing malicious software….