Any ideas on if it's possible to repair this fibreglass batten with glass/carbon? Or am I wasting my time. It's broken just at the cam.

Thanks

Use gaffa tape and if that doesn't work, just use more gaffa tape.

Did something very similar recently

I got an old matching piece from the local windsurfing shop ($5)
Cut to suit length
Affix with something like knead-it or some other putty. let it harden and sand down
cut fiber glass lengths approx 20cm long & mix your resin and catalyst (I just used a surfboard ding kit i had)
soak the glass strips in the resin to get completely wet
Wrap broken area up overlapping about 5-7cm either side of join
use electrical tape to wrap up the glass and resin to squeeze out and excess resin
As the resin that comes through the tape starts hardening remove the tape
Sand to shape

This guy knows what he's doing:

www.seabreeze.com.au/forums/Windsurfing/General/Batten-Repair/

Or just buy a new end for the batten.