Twin-Star
Let's get this straight - there is no such thing as Night VFR. All night flights within the UK are IFR (or Special VFR in CAS) regardless of the weather conditions.
Therefore it is the Instrument Flight Rules one needs to refer to for night flight. The ANO states (from memory again) a minimum 800m in-flight visibility for night flight under VMC.
Although not relevant to this thread in any way, there hasn't just been talk of reducing the visibility limits for PPLs. The ANO was changed about a year ago restricting the minimum weather criteria for VFR flight. The minimum viz was set at 1500m regardless of type of licence held and the 'Clear of Cloud in Sight of Surface' was changed to 'Clear of Cloud with the Surface in Sight' which now has a clear definition and is more restrictive than previously. (Google CAP393 and take a look).
JJ