In the case of an MGB with a hand brake warning light, engine cranking erroneously with ignition switch off can be caused by the brake warning circuit diode going short-circuit. This causes the engine to crank if the handbrake is on. i.e. if you drop the handbrake the engine should stop cranking. This diode is mounted on the bulkhead behind the glovebox and has one white/red on one side and two green/orange on the other. The current is supposed to flow from the white/red to the green/orange.