Hi Graham,
I have the same scenario (some of my Therion projects import a Survex 3d file to define the centreline. The easiest solution is to use the version switch when generating the 3d files to generate the 3d file version which Therion can read. I wrote a batch file with the following line to do this for my main project:
"C:\Program Files (x86)\Survex\cavern.exe" -v7 mycavedata.svx
Other more technical solutions may enable Therion to read the newer format 3D files (I expect Wookey is already typing something about the Debian version as I write this!)
Footleg