Morning BOAC..
CTR TK PUMPS go OFF during procedure, you then monitor wing tank quantities.. faster depletion on one side by >300kg in 30 minutes indicates a leak.. shut down the engine on the leaking side, if the leak stops it's the engine, if not it may be the wing tank.
CTR TK PUMPS go back ON after shutting down the engine to restore normal fuel feed - they feed the engine not the tank - so I will now edit my previous post!