The problem is that anchors created within a particular topic only display as possible anchor links in the drop down menu when adding a link from within the same topic.
I could not create a link to an anchor from a different topic using the drop down list. The anchors for the other topics did not appear.
I wanted a 'Contents List' for class topics, i.e. have links to each class topic all within topic zero. Using the page reference followed by #section-1 as per advice above did work though.
Thanks
cb