Viewers knew as soon as she spoke that she was a wicked woman. In television drama, if a member of an ethnic minority says that a character is a racist then she is a racist.  Sure enough, the murderer bumped her off and Lewis discovered that she was a blackmailer who used incriminating information she had collected on the force to extort money from her victims.

If I had black or brown skin, I might welcome media executives, who saw me as virtuous by definition, and admire their determination to oppose those who would oppress me. I might then register that for all their fine sentiments, they did not allow anyone who looked like me on to the screen. A black actor played a doctor Lewis visited, but had just one line. In a certain light and from certain camera angles, one of the glamorous women returning to the college looked as she may have come from an Asian family. I had to stare at her so long and hard to decide if she was white or not, I worried I was developing the obsessiveness of an Apartheid-era policeman — never a healthy symptom — and turned away.

Media people hate the accusation that they deny work to ethnic minority actors as a matter of course. ITV executives said they were "shocked and appalled," by the boast of Brian True-May, the creator of Midsomer Murders that he wanted to keep them out of his "last bastion of Englishness". They required him to resign, but their outrage was synthetic. True-May could not take casting decisions alone. In any case, the black exclusion zone does not just cover Midsomer but virtually all historical dramas along with Miss Marple, Poirot, Kingdom, Doc Martin and, as we have seen, Lewis. The middlebrow hits that draw audiences of 8 million, and whose makers sell them overseas as representations of "quintessential Englishness" are all white, except for the subordinate roles. Commissioning editors might say that they cannot hire ethnic minority actors for historical dramas — although the RSC and National Theatre have practised colour-blind casting for years — but how can they justify excluding them from contemporary stories?

When True-May's superiors continued that his sentiments were "absolutely not shared by anyone at ITV," they inadvertently provided an explanation. The dominant culture in broadcasting is not True-May's creepy notions of Englishness but an unthinking liberalism that combines political correctness with a colour bar. They love the ethnic minorities in theory, but don't give them work in practice.

I cannot imagine a British television writer producing a script in which a black police officer made a false accusation against a white colleague to settle a score, or writing a comedy about the hypersensitivity on matters racial in the workplace. Nor can I imagine a thriller in which the murderer was a black man, unless, of course, white racism pushed him to kill. Producers do not think that they are denying black or Asian actors work as villains or as any other tainted character. They think, when they think about it all, which I am sure is not often, that they are combating racism by refusing to use plot lines that might appeal to prejudice, real or imagined.