Thanks for the previous help which pointed me in the right direction.
I carefully reread the documentation and worked it out. Screen shots attached. A user registers then can select an entity form to submit info, the entity has a relationship with contacts. The inserted record carries the relationship.
The user can then further update their contact record. When logged in these settings retrieve the existing contact.
General setting. Entity name = Contact, Form name = contact, tab Name Details_Tab, Mode EDIT, 'Current portal user', your website
Entity Reference. Set..on save YES, Entity logical name = Contact, relationshop name contact_customer_details, Entity Reference Source 'Record Associated to Current User'
cont. Record source relationship name contact_customer_details.
I was so pleased when it started working!