Just wondering if at the flight planning stage say there is an INTER at the destination reducing vis to below minima, and I decide to plan 30mins holding (assume it is sufficient to last through the period including 30min buffers) does that mean I include the 15% VR with this 30min fuel figure?

What about if I stipulate an alternate instead? Do I add 15% to this (possible) increase in flight time?

The variable reserve is intended to cater for delays that may happen during the flight from the departure aerodrome to the destination. Such things as unexpected headwinds or diversions around weather or bushfires.

Holding fuel is carried to simply hold and wait for the weather to improve, [over some visual fix in the case of VFR airacraft, or over the destnation aid in the case of IFR]. Therefore headwinds or diversions are not considered. You never need to carry 15% of any holding fuel, it is treated as just an extra fixed reserve.

However if you are planning for the possibility of proceding to an alternate aerodrome, that is treated as just another flight and headwinds and/or diversions are considered. You always include the variable reserve for alternate fuel.