Thanks for the feedback guys. As mentioned I've repaired (and shaped) quite a few surfboard, so I took the same approach with my sailboard.
As usual, once I started picking away the cracked carbon fiber, the hole ended up being a fairly decent size.
My board's black and even with the "breather hole" unscrewed my problem were bubbles coming through my mix... lots of bubbles

Maybe I'd done something wrong?? So once it cured, I dug out and started again.
This time though, I bogged the (now even bigger) hole with MARINE epoxy putty. It's a two-part epoxy putty which is kneaded in your hands and hardens in a matter of minutes once mixed. Great stuff!!!! It allowed me to really push the putty (like play dough consistency) into all the cavities and no air bubbles were able to push through.
I then scraped off the excess and used Gladwrap to mold the putty to pretty much the exact shape of the nose and rail, which meant very little sanding. I still sanded it back once dry and finally applied a coat of Epoxy resign.
I didn't apply any matting, so I hope the hole (pun) thing stays together.
Finally, a coat of black spray paint was applied and it looks brand new. Apart from the shiny paint compared to the mat, you'd never tell there was a hole there.
So anyway, MARINE putty is really awesome!!! I'll definitely use it next time. I'll forever use it instead of q-cell and, if necessary, apply glass on top, but I don't think it needed it this time.
Peace
Nige