Contact Summary Layout Extension - Screen display problem
I was setting this up in a staging environment which is not up to date with the latest versions of CiviCRM. If you go are running a version of CiviCRM earlier than 5.24 then you need to apply this patch which solves the problem. The issue is not seen in versions of CiviCRM 5.25 and later.
(https://patch-diff.githubusercontent.com/raw/civicrm/org.civicrm.contactlayout/pull/79.diff)