As promised, my ethnic background is "human".
There was a box to explain briefly what I did at work. That is impossible, and patent attorneys are notoriously bad at explaining anything briefly. I had just enough room in the box to put "Explaining things at much greater le", though.
My form had a printing error; there was a bit missing. So I've been helpful; I wrote in for them "No, I don't speak Welsh".
Finally, they wanted a phone number in case they had any problems. Again, I've tried to be helpful. There was a note on the front of the form giving details of a helpline in case you had problems with the census form, so I wrote that in for them.
Job done, I think.