This break happened a year or so ago and I repaired the chair by getting some glue (from Mitre 10, as recommended by a staff member to do the job) and glueing the chunk of wood that had split from the leg back onto the leg, then glueing the leg itself to the chair and tightening the screw.
This repair job lasted ok till this morning when the leg fell off again.
I think I now need to get it professionally repaired. It's only a cheap dining room set but the chances of finding a replacement chair identical to the rest are slim.
Since my woodworking skills aren't all that great, can anyone recommended anywhere that does this sort of work on the North Shore in Auckland? The Yellow Pages gave me some upholsters and furniture restoration people, but I think this might be beneath them!