It’s strange that, in television drama, the hidebound, rule-driven world of the law has become the last bastion of the maverick, the risk-taker and the loose cannon. Television police officers, as in something like the ­excellent Line of Duty, are now aware of “best practices”, even as they subvert them; ­barristers, in contrast, seem able to be as ­spontaneous and emotional as they want, just like they used to be in Rumpole of the Bailey.At least, they can if they are Martha Costello in Silk (24 February). Martha (Maxine Peake), a battling QC, is all heart. We knew that from previous series, but it was rammed home here in the opening minutes when her client lost his appeal. Martha did not stand when the judges left, then angrily smashed the water decanter in