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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, 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 ]
    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))
    
    qgis.utils.iface.legendInterface().refreshLayerSymbology(layer)
    canvas = qgis.utils.iface.mapCanvas()
    canvas.refresh()