OK, so I am feeling very stupid, but, I am betting others have had a similar experience. So here goes:
I just spent several minutes trying to work out why my carpenter could not work. He simply stood taking planks out of and putting them back into the plank’s stockpile.
The answer was really simple, I had placed my workbench facing the wall of the shelter. I have noticed that on some objects this is not immediately obvious. Perhaps object directional arrows would be useful if they are not too difficult to implement.