I took a photo of how I fit the cable tie to keep the plastic "wall" or "tongue" in place , stops the gearbox from moving and allowing the belt to slip;
(http://i74.photobucket.com/albums/i257/1stbooboo/c5restoration001.jpg)
(http://i74.photobucket.com/albums/i257/1stbooboo/c5restoration002.jpg)
Another piccie (taken at retroAndys Royal Run) , a good strong solution to the problem. The cut off end of the cable tie only just misses the large white drive cog so probably best to secure it further along as in the earlier pictures.
(http://i74.photobucket.com/albums/i257/1stbooboo/andysrun12jun11001.jpg)