Some jobs are really too difficult for immigrants, depending as they do on a deep understanding of British life and the nuances of the English language. Or at least, that is what we thought before the arrival of the German comedian Henning Wehn.In the last few years, the heavily-accented stand-up has become a fixture on BBC radio and television, often making use of his outsider’s perspective on our way of life. This week he was an excellent presenter for An Immigrant’s Guide to Britain (3 May), which, while purporting to be a public information film for newcomers on behalf of the British Immigration Office, was actually a series of comedy encounters and sketches designed to make us laugh at ourselves.In short order, we learned that Britons have no idea about the general knowle