Hi Mick,
Yes, you are correct. The predecessors column on a workitem item upload, creates dependency FTS type with no lag. For more sophisticated dependency options, you would need to load it separately with dependency links.
See screenshot: http://screencast.com/t/fxZ5wM2wn4Yr select Item Type > Dependency link > click on Fields List to see fields Lag and Dependency Type columns available for update: http://screencast.com/t/JVlhA5m3U19
All the best,
Diana