Roof joist insulation with ceiling joist insulation?

I have a partially boarded attic with typically 300mm of insulation between the ceiling joists. This provides good insulation for the habitable rooms below. I'd like to add a small amount of insulation top the roof joists, mainly to stop the attic temperature from plunging in the winter. Does anyone here have any advice on this?

I'm aware that I need to leave a ventilation gap between the roof joist insulation and the roof, and to leave ventilation gaps at the eaves. My thinking was to put 25mm rigid batts resting on boards covering the roof joists. How much of an air gap between the batts and the underside of the roof should I leave? The alternative is to just plasterboard the roof joists: if I use plasterboard with a silver backing, would that make much of a difference?

The attic is used for storage only.