Take the M4 to junction 43, leave the motorway and follow the A465 to Hirwaun.
Then take the A4059 to Brecon, join the A470 towards Brecon.
At Brecon centre (just after the bridge) at lights take second left, follow around to next lights. Go left, then take next left up Priory Hill, A4520.
Follow the A4520 untill just after Upper Chapel. Then take A4519.
At the bottom of the A4519 go left on the A483 towards Llandovery.
At the T jungtion go left then right on to the A4069 to Llangadog.
At Llangadog Town centre go left and follow the A4069 up over the mountain & enjoy.
When you get to Brynaman follow the signs to Pontardawe then follow the signs back to the M4.
*This was borrowed from a well known motoring website - hopefully the author won't be too annoyed if they find this version*