Lamp flickers when dimmed

My arc lamp flickers whenever I turn the dimmer below half. The bulb is an LED one from the shop. Is it the bulb or the dimmer?

Most likely the dimmer. Older dimmers expect a bigger load than one LED bulb draws. A trailing-edge dimmer usually fixes it.

Swapped the dimmer at the weekend and it's perfect now, thanks. No flicker at all, even at the lowest setting.