Thanks hansmex.. I'd tried all that stuff already and copying into a new directory.
In the end it was something really silly.
Even though I was in the directory holding the sh3p file, I needed to specify explicitly the directory, not just relative.
ie.
/usr/bin/sweethome3d /home/joebloggs/downloads ...

I'm running on Ubuntu, every time I try to install a plugin I get the following error pop-up from sh3d.

Running command: /usr/bin/sweethome3d ExportToHTML5-1.1.sh3p

Getting "Can't import plug-in"
pop up in dialogue box.

I've also tried manually moving the extract into ~/.eteks/sweethome3d/plugins ...