Info from the guy who runs the Merthyr site:
The M5 is the quickest. Don't follow any of the routes you have been given. This one is not as the crow flies, but the easiest not to get lost by.
M5, M50, A48, A465......
Pass Merthyr (follow the signs of Neath) until you come to the roundabout where the A470 intersects the A465 (signposted - <Cardiff - Brecon>). Turn left towards Cardiff.
At the next roundabout - left at the McDonalds
Next Roundabout straight on.
1st Traffic lights - straight on.
2nd Traffic lights - take a left and drink in the Lantern. You can park by the pub. The ground is about 200 yards behind the Lantern - follow the sigh posts. Do not drink in the Park View in the street above - it is RAF (rough as .....
Have a good trip.
Mike D