AIP GEN 3.3
4.10 For Aircraft Flown at Night Under the VFR: the area to be considered must be:
a. the area specified in para 4.8 or 4.9 for aircraft navigated by means of a radio navigation system; or
b. within a radius of 10NM from any point along the aircraft’s nominal track.
However, the pilot of an aircraft who has positively determined by visual fix that a critical obstruction has been passed may nevertheless descend immediately to a lower altitude, provided that the required obstacle clearance above significant obstructions ahead of the aircraft is maintained.
Note that whilst you are permitted to use navaids, position fixing for enroute descent as per the quoted paragraph must still be visually confirmed.
Ps, for other position fixes as per ENR 1.1 Paras 19.4 & 19.5, the AIP do not specify IFR only.
