I have now got this sorted thanks.
Official response from HelloOx makers said that doing any kind of reset 3 * Green or *#7370# can stop it from working. they advised to re-install the latest firmware from Nokia Software Updater to get working.
Here's the process i used to fix my problem: (memory card unplugged throughout process)
1) Backup of contacts etc already done. (do them if not)
2) Re-install firmware 400.21.013 via Nokia Software Updater
3) When reinstalled - access internet via the built-in E71 web browser and download and install HelloOx 2.03 (signed)
4) Run HelloOx from the Menu / Installations
5) HelloOx now gets past the "Unpacking File..." section but hangs on "Installing Cert or Installing Root Cert" (don't panic - i found the solution)
6) Close the HelloOx app, but don't uninstall it.
7) access internet again via the built-in E71 web browser and download and install HelloOx 2.03 (signed) - When asked if you want to Overwrite previous version - say ok / yes.
8) Once you have re-installed HelloOx again, run HelloOx again and this time the install will complete successfully!
I hope this guide helps save someone else 6 hours of trial and error! Thanks to everyone on here for there advice and to HelloOx team for their e-mail support.