Jeremy,
I've done the Flat Rock trip once myself having beach launched from Flinders.
It was a touch rough for the week we were there so only got out once. We picked up nothing of size around the rocks but plenty under - heaps of bait here if you want to pick up some slimies for trolling.
I've got some GPS points for about 12 ks past the headland if you're interested. I've fished there once before and nailed them on a friend's boat but suffered too much in the swell the time I went out.
Let me know if they're a help to you.
bugman