County councillors in England are elected in units called CountyElectoralDivisions (CEDs). These are usually, but not always, coterminous with whole numbers of wards. When they aren't, they are usually, but not always, coterminous with whole numbers of parishes. Basically it is a mess. For instance, here's ![[WWW]](/wiki/mstheme/img/moin-www.png){kind=small}
a map of Norfolk (2MB PDF) showing its division into parishes, wards, and CEDs (though in that case I think all the CEDs are whole numbers of wards).
The key point about CEDs is that they are not listed in the AllFieldsPostcodeDirectory (because the OfficeOfNationalStatistics doesn't compile any statistics at the CED level). This is a nuisance.
The BoundaryCommitteeForEngland is responsible for maintaining the division of counties into CEDs. It produces reports and maps detailing the assignments, but doesn't appear to have a convenient machine-readable version of the data.
(I spoke to Christopher Morgan, cmorgan@..., about this.)
(Note that even UpMyStreet's "my representatives" page doesn't get county councillors right, at least in my area --ChrisLightfoot.)
Another suggestion is to use a product from Experian (the credit referencing people) which connects electoral roll entries to postcode and, apparently, CED data. I haven't looked into this yet.