During Raymond Mason’s 20 years of acting in the ITV soap opera Crossroads, he played five different roles. “I don’t think anyone ever noticed,” he said, “and I don’t put it down to versatility.” For trained actors in the days when there was just a handful of drama colleges and fewer vocational courses, the pool of talent was by definition smaller. Many performers found themselves appearing more than once in the same programmes. For Raymond, the roles that he played on British television over 40-odd years numbered more than 1,000, and he appea…