Ive reflashed back to 6b1 which was the version I was previously using and had no problems with.
Reminder to others: I lost all settings by downgrading the f/w and had to reconfigure all settings again. So warning to have all info to hand if you are going to do this.
Ive also spoken to Zyxel and reported this. They are going to put a request back in to HQ to re-instate multiple telnet sessions.
I've only had issues with 6b5 so I reflashed back to 6b4.
I was on 6b4 which was the version which locked me out, so I hope that all is ok for you on 6b4 or you may have to down grade further.