Complex sentence: The leader of the gang was Howard Wilson, a former police officer who had resigned disillusioned at his lack of promotion and now had debt problems, and the others were John Sim, a policeman-turned-salesman, and Ian Donaldson, a car mechanic.
Split into simpler sentences: The leader of the gang was Howard Wilson, a former police officer who had resigned, disillusioned at his lack of promotion and now indebted. The others were John Sim, a policeman-turned-salesman, and Ian Donaldson, a car mechanic.