Valuations office say I don't exist!!

ditsydoris_2

According to the Valuations office my mothers address in Finchley North London does not exist! I know for a fact it does, and i also know that we lived there in 1991. My mother only moved 2 years ago so where do I go from here? Everytime I enter the address to check on the council tax banding it tells me no address exists. Does this mean we shouldn't have paid any council tax? Or is it a ploy to stop us trying to find out what the rest of the road were paying at the time? We know of a bigger house that was paying less for a while so where do we go from here? I entered my own address in Barnet which has not changed in Valuation from 1991 until now so I know the service works! Any ideas anyone? Or can we claim for paying council tax for a property that apparently does not exist?