Not knowing the history of the mast or the spreader I would suggest that firstly you need to give both parts a jolly good rubbing down with some sand paper to ensure that they are going to bond well. If they have been lacquered, then keep sanding till you get some carbon dust coming off so that you know you have gone far enough.
Next, you need to bond the spreader back onto the mast. To do this I would suggest mixing up some epoxy with some microfibres and colloidal silica to give you a decent thick paste. Smear this on under the spreader and then splat the spreader into position. Using the end of a Magnum ice cream stick (other brands are available) make a nice fillet all round the join. Let this go off and then sand it all nice and smooth to remove any lumps. Now you want to add some carbon fibre to add some additional strength. Wet out the whole area with epoxy and then start layering up some weave over the join, probably a good 2-3 inches either side onto the mast and the spreader to ensure that you get a decent bond. Not sure how many layers you'll need, but if you start small and get bigger as you put them on it should spread the load nicely. Also, if you put them on at different angles (so the weave fibres aren't all going the same way) then it should be a bit more robust. Once you have decided you have enough on there (in this situation, having had it break before I'd suggest going to town on it so you can worry about sailing more rather than fixing spreaders) then apply your peel ply. Wrap it tightly to make sure it is neatly compressed and then let it go off for a while in the sun or somewhere warm.
Once it has cured, peel off the peel ply in that satisfying tearing event (maybe even do it in the dark and look at the sparks fly!) get the sand paper out again and get it all nice and smooth, then apply paint / laquer or whatever.
Have a reward beer, then go sailing.
