Don't know if this will help but you can appeal a Council Tax Banding
if you find out that other similar properties in the area are classed in a lower banding.
i.e. if your property is classified in, say, Band E and you find out that other similar properties are classified in Band D you can appeal for your property to be classed in the lower banding - this would have the effect of lowering your Council Tax by default.
Some years ago I lived in a one-bedroomed flat (small block of 12 flats) which was in Class C; one of the residents found out that similar properties in the area were in Class B and appealed; the appeal was upheld and we all got a refund backdated over a number of years; if I remember correctly I got total refund of about £700