Thanks John, I had a look at: https://bugzilla.suse.com/show_bug.cgi?id=987487 and saw that, following your comment, Fabian Vogt had added one to say that the latest fixed version of sddm was now in the main repo. I therefore thought it best to do another "zypper dup" (1000+ items) before trying your workaround (even though I already had sddm-0.18.0-7.1 installed). Upon reboot sddm is working again (without your workaround).