I've worked out how to do this now so here's my method for the record.
# create 1st sector - use the **ori** parameter to control what bearing it
# starts on (0 is vertical)
rad1 = visual.RadialStim( win=win, name='rad1', color=[1,-1,-1],
angularCycles = 0, radialCycles = 0, radialPhase = 0.5, colorSpace = 'rgb',
ori= -90.0, pos=(0.5, -0.3), size=(0.3,0.3), visibleWedge=(0.0, 135.0) )
rad1.draw()
#now draw another sector next to it by using **ori** again to line them up
rad2 = visual.RadialStim( win=win, name='rad1', color=[-1,1,-1],
angularCycles = 0, radialCycles = 0, radialPhase = 0.5, colorSpace = 'rgb',
ori= 45.0, pos=(0.5, -0.3), size=(0.3,0.3), visibleWedge=(0.0, 45.0) )
rad2.draw()
This works fine in Builder, dropping it into a code segment and knowing that Builder uses a window called win
Note that this uses the default mode for size (-1 to 1) so it will look different on different screen sizes. you may want to change to cm
(and calibrate your monitor) if you want it to be round.
Thanks to Jon P for his suggestion (via google groups) to use RadialStim)