Which dimmer do I use with this fitting?

Any of the Varilight dimmers will work for you, look at those here: https://www.lampshoponline.com/lighting-accessories/dimmer-switches.html?brand_final=1462 Alternatively, any dimmer switch that works on 'trailing edge' dimming will work for these downlights. Explore what we offer here: https://www.lampshoponline.com/lighting-accessories/dimmer-switches.html