Challenging Council Tax Band After Moved

I'd be really interested if anyone else has been in this situation and how they got around it...On moving out of our property (New build in 2003) in September, we realised that all ourt neighbours in the same house type were in a Council Tax band lower than us. We have no extensions of improvements and sold prices of all our properties over this period are within £10-£20K of each other. I therefore believe I have sufficent evidence to prove we were placed in the wrong band. This has been the case since 2003. We realise we could be owed around £5K. I contacted the Valuations office in October, as soon as I realised, who said they won't correspond with me as I no longer live at the property, but they will review the property banding. They, therefore, won't tell me once this has been done, so I then don't know when to approach the council for a refund. The property is currently unoccupied as we sold it in a part-exchange with our new developer who is in the process of selling it on. My questions are these: How long does the process of reviewing a banding and correcting it, typically take? Do I have any right to appeal if the band remains the same and I no longer live in the property but can prove I overpaid for 15 years? I am worried that I will not get my refund on a techinicality that I no longer live there, albeit I only became aware of it on moving out. Thanks All!