For info - solved by the guys on uk.d-i-y....

It is a depth stop. Once I had the thing in
place I was amazed at how simple it was
to adjust and how well it worked.
Just slacken off the wingnuts and screw
the guide up or down as necessary.

http://www.oofus.com/pix/RBPics/Odds/GuideOnBit-1.jpg

http://www.oofus.com/pix/RBPics/Odds/GuideOnBit-2.jpg

http://www.toolsrules.com/ST49BG.JPG

Roy