Heavy Metal is one genre that does absolutely nothing for me, to the extent that I'm not really sure how it's even defined these days. A band called Y Trydan did a session for C2 a few years ago - they were rock verging on metal, I suppose.
In the past there have been a couple of notable art-metal acts - Y Rei and Ashokan, but again I'm not sure if they are what you're after.
Also, and you may find this difficult to believe, a Metal band called Eryr won the C2 battle of the Bands competition back in 2003. The drummer and vocalist/bassist later abandoned the group in order to concentrate on their younger brother's band - Cowbois Rhos Botwnnog!