there is some misunderstanding of the process in recent posts
a) the banding for the property was determined by the Valuation Office Agency (VOA) in 1991 or when the property was built (if after that) or when notified there has been a sale of a property with an improvement indicator registered against it. (The VOA is a branch of HMRC. It does however work with the separate Land Registry's records of who owns what so gets updated info that way)
b) the VOA notifies each council what the band is for each property in that council
c) the council has no say whatsoever in a banding decision
d) if you contest your banding, then it is the VOA who will examine the case and look at whatever evidence you submit
e) if you disagree with the VOA's contest decision you can appeal against the VOA and that appeal will be considered by a Valuation Tribunal which is independent of the VOA (and again has nothing whatsoever to do with your council)
Once the VOA has determined a band, or reached a decision in a contest, or been overruled by a Valuation Tribunal, then it is the VOA who instructs the council what band to use. The council must do what it is told, it cannot change anything.
Delays in the process are typically caused by the VOA having a backlog of cases from across the entirety of England (Wales, NI, & Scotland are different)