In fact, I was fishing a competition there with a mate about a month ago when he had a really shy bite which he thought would be a small whiting. When he started to retrieve it motored off under the pier. It looked like he might snap his rod at one point .. he just couldn't stop it. He reckoned it was a conger around the 14-15lb mark. So you do get the odd decent eel off there. Best place to try for them (so i'm told) is off either top corner casting into the deep water channel the waverly uses but you'll have to be there early to get either of those marks.
Ooops ! Almost forgot, next competition is Sunday 14th Novemeber, 5pm - 9pm from pier and beach (although most of the beach will be under water - 13m tide !. Booking from hut at the end of the pier.
Penarth SAC webpage is now at <http://homepage.ntlworld.com/haydn.matthews/>
You can find the competition dates and results from the last few years there.