It was the day after the May Bank Holiday when David (41) heard his doorbell ring. Again and again, the buzzer went, he remembers it clearly just before his dad kicked down the door. He had been bingeing on cocaine since the previous Wednesday, another relapse following years of attempting to stop. “I woke up to my adoptive parents, my biological father, two girls who worked in the bar down the road and two policemen in my bedroom,” he says. “The girls were taping up my arms where I’d slit them, and all I could think about was whispering to…