If you do decide to go to Brechfa, then give Cwm Rhaedr a try. its well worth the extra 10km or so to drive to.
http://www.forestry.gov.uk/pdf/CwmRhaeadrMtBkguide1.pdf/$FILE/CwmRhaeadrMtBkguide1.pdf
Cwm Rhaeadr is possibly the flowiest, nicest trail in Wales.*
(*Disclaimer – there maybe [probablly is] better but I’ve not tried them in my limited experience, and it will make you smile)
edit. about 7km total, 1/2 up then down, but so much fun you’ll want to do 2 laps. a little exposed at the top of the climb, but then pretty much all through the trees on the way down. loads of berms and bumps and jumps but all rollable.