FOUND THE PROBLEM, with the help of Nick at Graphisoft Support. The Line Category was set to 'Skin Separator Line'; when I change this to 'Drafting Line' my lines are visible. I guess the Skin Separator Line was being overridden due to Renovation Filter options.
Working with the Renovation Filter is new to me, and adds another level of complexity. Thanks All for your replies.
AC17 | iMac 3.5 i7 | OS 10.9