Main cast including Beckett and Castle (centre). Pics: Alibi   Rating ★★★½ Alibi, Wednesdays, 9pm, from 9 March  The recent vogue in UK television has been for crime series to become distressingly grim. An episode of Taggart recently featured three child murders, while the opening episode of Silent Witness included the rather gratuitous rape and murder of a child. The US series Castle…