In absence of any replies I thought I'd answer my own post in case anyone else searches for similar answers.
Parked in Abergorlech and linked up the Red (Gorlech) and Black (Raven) trails. Basically before the top of the last Red descent follow the Black arrows and you can't go wrong.
The Raven was really fun with some cracking descents. However I get the impression it could get hard work if it hasn't been dry - at least compared to trail surface of the Red run.
All of us were a little unfit and what I assume would have been the '1st' climb on the Raven from the Brechfa car park felt a little brutal with our legs getting a little tired by that point. I guess we could have mixed in a loop on the Green / Blue at this stage also if we were uber keen!
Can't believe both routes linked up only came to 22 miles on my Android GPS! Gotta get some fitness quick before the Dyfi!!!