roadlist=[]
layer = iface.activeLayer()
chainage_layer=QgsVectorLayer(r'Z:My DriveMangoesmappingSpatial Projects2019DSC126_Road_UpdatesWorkingOutputDSC_Roads_10m_Chainage.tab', "Chainage", "ogr")
for feature in layer.getFeatures():
    if feature['RoadMntnc'] not in roadlist:
        roadlist.append(feature['RoadMntnc'])

for road in roadlist:
    print (road)
    layer.selectByExpression('"RoadMntnc"= '{0}''.format(road), QgsVectorLayer.SetSelection)
    selection=layer.selectedFeatures()
    for feature in selection:
            chainage_layer.selectByExpression('"RoadMntnc"= '{0}''.format(road), QgsVectorLayer.SetSelection)
            chainage_selection=layer.selectedFeatures()
            if start_point(buffer(feature.geometry,5,10)).intersects(chainage_selection.geometry()):
                print (feature2['distance'])
        #aggregate(layer:='DSC_Roads_10m_Chainage.tab', aggregate:='max', expression:="distance" * (attribute(@parent, 'RoadMntnc') = attribute($currentfeature, 'RoadMntnc')),filter:= within(start_point(geometry(@parent)),buffer($geometry,1,10))) # process in model builder -field calculator.