Nicola Walker is one of those understated queens of TV – be it as a spy in Spooks, a conflicted wife in The Split, or a troubled detective in Unforgotten. So, no surprises that she’s quite something in new gig Annika, a thriller which follows DI Annika Strandhed (yes, another detective!), in Glasgow as she heads up the Marine Homicide Unit, solving the crimes and murders that wash up in Scotland’s waters – from the depths of the River Clyde to picturesque Loch Katrine. Intensely private, London-born Nicola, 51, who once shared a flat with her good pal, former Bake Off host Sue Perkins, at Cambridge University, is married to fellow actor Barnaby Kay, with whom she has a son, Harry, 15. Here, the down-to-earth star talks about getting into…