Neil,
I have been having a look through some of my documents relating to the Aero models, and have come across a Photoshopped copy of a drawing for the handlebar link. As you will see, the part number is 16867 - so it looks as if prewar spares do stock them! - Douglas have caused confusion by referring to it as a 'distance piece'! (I have changed the title to 'handlebar link' to make future referencing easier - see attached scan). You will also note that it is the second version of that drawing - I think the modification was the inclusion of a central slot to facilitate a smooth run for the control cables. The second scan shows the area around the top of the steering head - with the earlier, solid 'distance pieces'
Hope this is of some help,
Regards,
Eddie.