Hard to visualise without a diagram....what is this other '240v neutral' connected to?
Are you using a high impedance digital voltmeter? If so the capacitance from this neutral to an active wire could be high enough to produce 240v on no load....especially if the neutral is part of a long length of twin cable.
If this '240v neutral' is the return for some other load which is switched on then obvioulsy it will be at 240v when disconnected.
