I don't have anything to add regarding what PP has said about statute of limitations and the other advice he gave at 07.49hrs. Legal info on here runs a lot less than £200/hr.
The other suggestion I could make at this time involves researching for yourself the ratings history for your property. It might help you when it comes to understanding the history of how it got to the present muddle. Property rating is the business of the Valuations Office Agency (VOA) in England & Wales, and it (should) work in close partnership with the local authority. It is the VOA that decides what council tax band, or what RV (for non-domestic rates on commercial property) is applied. They get tipped off by the local authority when (for example) someone improves their property by planning development, or when a property changes from being a commercial to a domestic property. It is this facet you might want to check out, because it may give a clue about what has (or hasn't) happened. If the original property was a farm, I would expect the VOA to record that as a 'mixed use' or 'composite' property. What this means is that it has both domestic (payment of CT) and commercial (payment of non-domestic rates) obligations. A holiday let is a bit of a strange one. It is regarded as a commercial property if let on several short-term lets totalling more than 140 days per year. Less than this and it should have a domestic (CT only) liability. It is imcumbent on the owner to advise if a long-term tenancy (as you have) is established, whence CT only is payable. It is perfectly possible that a holiday let CAN be let as an AST for an extended period of years, provided it is permitted within the planning consent conditions granted. Different councils appear to have different rules on this.
The CT (domestic) part of the VOA website is accessed here:
https://www.gov.uk/council-tax-bands
(it is accessed from the central government gov.uk website family)
You input your postcode, and it will show all the domestic properties in that code and their current band. Click on a property and it shows the single property, from which you can see if it is 'mixed' use, and access the history of the property back from when CT started (remember the poll tax fiasco before it?).
Non-domestic rates enquiries on the VOA website work in the similar way but the starting point is here:
https://www.gov.uk/correct-your-business-rates
It accesses another database within the VOA portal. Again the starting point is the postcode. The output is presented as an RV, and again you have access to the complete history - certainly back to pre-2000.
A mixed use property will have entries in both sections of the website.
Why am I suggesting this work? Because forewarned is fore-armed. In theory the local authority has this historical record also (each authority in their finance section accesses and maintains data from the VOA to be able to collect business rates and council tax) but as was discussed earlier, its better to sit tight and see what the CT collection people come back with regarding your unfortunate situation. DB