Using the beta 606 of CC; the software has produced a much better result.
Lots of potential now for illuminating the glass from the side, adding a wooden frame and standing the whole thing on a base. I now am interested in seeing what I can do with coloured glass, frosted glass and assembling panels of pieces. Lots of artistic potential here.
Feed rate was 400mm per minute and depth of engraving was set at 0.050mm. Touch off manually and no BitZero or Bitsetter involvement. Piece required about 1 hour and 45 minutes to cut entirely. I bathed the glass in CT 90 cutting and tapping fluid which stayed where it was put because it is reasonably viscous.
The retract height was left at 16mm because I was unsure whether CC would try and break the bit by lanching it across the face of the glass, into a clamp.
I would like to thank @Ed.E for his welcome assistance in solving this issue for me. Thank you Ed. I very much appreciated your invaluable help.