To the survey whizzes out there. Ultimately I'm trying to end up with an .STL to play with.

With the software I have available to me, the only route I can currently see is to take a .VRML from Therion into my CAD software for manipulation. I have so far failed to get the Therion command working, even though it should be a simple one liner!

Alternatively I'm sure I could take the raw data points out as a .CSV and importing, but I'd like to see if there is a less 'manual' method first.

Any help appreciated

It is probably best to use the survex https://survex.com/maillist.html and/or therion https://therion.speleo.sk/contact.php list for this, as the emails go world wide to the users, plus makes a very useful resource to search. I only saws this post by accident

So you can do a dump of a 3d file to get something that looks very like a xyz file, which with very simple editing can be changed to your preferred point cloud, XYZ,SVY,PTS,PTX,TXT,PLY or many others. You then need to use your preferred software to reconstruct a STL, to be honest I did not find it very satisfactory, and only really works if you have splays,

VRML worked for me from Therion I used

export model  -o PPH2th.vrml

The Therion website has something I have not tried but it might help

https://therion.speleo.sk/wiki/examples#models

Best of luck