This link might help you:
http://www.voa.gov.uk/council_tax/cti_home.htm
and where it says:
Property values and bands
The council tax band of a property is not related to its current market value. This is because, by law, council tax valuations are based on the price a property would have fetched if it had been sold on 1 April 1991. For Wales the Valuation date is 1 April 2003. General price movements in the housing market since that date are not, therefore, a reason for changing a council tax banding. If you are thinking of challenging your band, please read the 'Can I challenge the list entry?' pages for the circumstances that permit a 'proposal for alteration of a list'.
Therefore all based on the 'assumed' value on 1/4/91, irrespective of the fact it might be brand new. So if you can find out what comparable flats were sold for then, and hopefully they would be in Band B, or even better band A, a challenge could well be worthwhile.