Quick update on this problem.
I’ve re-installed the version of WishList that was there before the update went wrong and the problem has gone away, although the plugin page is asking me to upgrade the plugin again.
From a quick code review it seems the latest version of the plugin has code to check for WooCommerce and the previous version doesn’t.
As mentioned, WooCommerce is installed and working and so I still don’t know why the upgrade of woishlist plugin doesn’t work.