As far as I am aware there are no trails beyond fireroads in Hiraethog Forest and none are specific bike routes.
Cloclanog (sp?) has some trails but not sure what they are like.
The Clwyds are somewhere I keep meaning to ride as there looks to be some interesting stuff – must remember to stop driving through on the way to somewhere else.
Quick google brings up this:
Clwyds
Must be some locals on here somewhere.