This certainly seems to be puzzling people!
I think I may have found the answer though (finally) thanks to signing up to Belgocontrol and being able to look at their AIP:
8.1 VFR flights at night
In Belgium
Balloons, gliders, DPM and ULM exempted, VFR FLTs at night are authorized from and to agreed airfields approved for night OPS:
Within controlled airspace:
in respect of the CLRs provided by the involved ATC unit
with transponder equipped ACFT
Outside controlled airspace:
between 1 000 ft AGL and 4500 ft AMSL on the regional QNH and, above 3 000 ft AGL, at a level in accordance with the semi-circular VFR cruising table
in two-way radio contact with the unit providing FIS
only with transponder equipped ACFT
So it seems that I can do night VFR in Belgium as long as I meet those criteria (which I would do anyway).
So what I need to do is file a flightplan with Z marked in flight rules for VFR to IFR.
Unless there's something else I've missed...this seems to go on and on!