Hi Don, many thanks for the speedy reply.
I build a Mercator projection, and set its defaultMapArea with a ProjectionRect (cartesian values, i.e. projected from the 
lat,lon).   I then construct a MapProjectionDisplay and call setMapProjection.
I have got it all working.  This was actually a sort of red herring.  I wasn't seeing any Displayables show up, and I was 
erroneously assuming that my projection logic was wrong.  So I looked at the main method in the MapProjectionDisplay (was a life 
saver, thanks for including it!).    I wasn't ever calling the 'draw' method, doh!
Stuart