I'm sure vernon's right, but in the other hand I've found through bitter experience On other routes that if you miss a sign, because you're daydreaming or whatever, then you do need a map to get back to the route.
Anyway this looks like it might be what you want:
http://www.openstreetmap.org/user/Richard/traces/2201
If not, try googling 'taff trail gpx' there seemed to be other possible candidates.
Depending on what GPS you have, you may need to simplify the track down to 500 trackpoints. Post again n if you need help with this.
You may also find that this is useful:
http://richard.dev.openstreetmap.org/garmin/gmapsupp.img
The referring link says it is a Garmin .img file of the NCN (or much of it). I can't download to check it as I'm on an iPod and is a 365mb file.