Very useful Duncan, and something I never knew you could do in Survex.
I've updated the instructions for my cave converter tool to suggest this is the way to convert .3d files to .dxf as it appears that the coordinates are not only re-centred around the origin by Aven when exporting as DXF, but also the cave length is not the same afterwards. So some sort of error is creeping in to the DXF files generated from Aven. The ones generated using the 'Convert to DXF' menu appear the same length as the original however.
I've also fixed a bug in the DXF conversion mode of my cave converter as this was also rounding all station positions to the nearest metre during import, giving rise to significant rounding errors. :-[
This bug only affected conversions from DXF format to other formats.
I've uploaded a new version to the usual place:
Cave Survey Data Converter