I haven't been able to figure out how to fix the bindings if this was indeed the cause, I've used a work around, which was to follow the approach presented by [pyqgis-cookbook][1], and to remove the existing ranges / symbology and re-create new ones (based on the existing ones). This approach now works for me. I'm not sure how efficient it is, but with the data set I'm currently using, it's quick enough.

    from PyQt4.QtGui import QColor				# import this namespace as I want to refer to QColor later
    from PyQt4.QtCore import QVariant
    
    def satramp(hue):
    	layer = qgis.utils.iface.activeLayer()
    	grads = len(layer.rendererV2().symbols())
    	vinc = 0
    	val = 180
    	i = 0 - (255 / grads)										# used to increment the saturation value
    	j = val - (grads * vinc)
    	for sym in layer.rendererV2().symbols():		# loop through each symbol in the layer using 'new symbology'
    		i += (255 / grads)
    		j += vinc
    		sym.setColor(QColor.fromHsv(hue,i,j))
    	canvas = qgis.utils.iface.mapCanvas()
    	canvas.refresh()
    
    def rap():
    	canvas = qgis.utils.iface.mapCanvas()
    	layer = qgis.utils.iface.activeLayer()
    	layer.reload()
    	canvas.refresh()
    
    # gets the layer for the passed index value based on the legend index
    def getLayerByIndex(layerIndex):
    	idx = 0
    	for layer in qgis.utils.iface.legendInterface().layers():
    		if idx == layerIndex:
    			break
    		idx += 1
    	return layer
    
    # gets the field index for the given field name
    def getFieldIndex(fieldName,layer):
    	mFieldMap = {}
    	fields = layer.pendingFields()
    	for ( key, field ) in fields.iteritems():
    		if field.type() == QVariant.Int or field.type() == QVariant.Double:
    			if field.name() == fieldName:
    				return key
    
    # sets a saturation ramp for a graduated layer only, based on the passed hue, value and index of the layer from the legend
    def satramp(layerindex, hue, value):
    	layer = getLayerByIndex(layerindex)
    	grads = len(layer.rendererV2().symbols())					# determine the number of graduations to use based on 
    	vinc = 0													# value incrementer, currently not used, but can be adjusted to increment the value also
    	val = value													
    	i = 0 - (255 / grads)										# used to increment the saturation value
    	j = val - (grads * vinc)
    	layer.reload()												# refresh the data source
    	
    	# determine range increments
    	provider = layer.dataProvider()
    	fieldName = layer.rendererV2().classAttribute()
    	fieldIndex = getFieldIndex(fieldName,layer)
    	min = provider.minimumValue(fieldIndex).toDouble()[ 0 ]		# get the min and max values to calculate the new gradations
    	max = provider.maximumValue(fieldIndex).toDouble()[ 0 ]
    	print max
    	rangeCount = len(layer.rendererV2().ranges())
    	rangeIncrement = (max - min) / rangeCount
    	rangePoint = min
    	
    	# construt a new renderer using ranges based on the existing renderer
    	rangeList = []	
    	for range in layer.rendererV2().ranges():
    		newSymbol = QgsSymbolV2.defaultSymbol(layer.geometryType())
    		label = str(rangePoint) + " - " + str(rangePoint + rangeIncrement)
    		newRange = QgsRendererRangeV2(rangePoint,rangePoint + rangeIncrement, newSymbol, label)
    		rangeList.append(newRange)
    		rangePoint += rangeIncrement
    	
    	newRenderer = QgsGraduatedSymbolRendererV2('',rangeList)
    	newRenderer.setClassAttribute(layer.rendererV2().classAttribute())
    	newRenderer.setMode(QgsGraduatedSymbolRendererV2.EqualInterval)
    	layer.setRendererV2(newRenderer)
    	
    	for sym in layer.rendererV2().symbols():		# loop through each symbol in the layer using 'new symbology'
    		i += (255 / grads)
    		j += vinc
    		sym.setColor(QColor.fromHsv(hue,i,j))
    	
    	canvas = qgis.utils.iface.mapCanvas()
    	canvas.refresh()


  [1]: http://www.qgis.org/pyqgis-cookbook/vector.html#appearance-symbology-of-vector-layers