Council Tax and Business rates

[learning110]

Good Morning,
Just over a year ago, we completed on a purchase of a cottage to be converted into a holiday let. The initial aim was to get it ready within six months, but with the pandemic, contractor availability and lack of supplies, it took just over a year. The project is now complete and working well.

When taking on, I notified the local authority that the cottage would be used as a holiday let and therefore commercial and needed to be moved from council tax to business rates. The local authority told me it needed to be on the market for to let before they could make the change, so reluctantly, I paid the council tax charges. As soon as the new financial year started the council sent me a letter stating that because the property had been empty for a long period (much of it when it was on the market for sale), they were doubling the council tax. After reading various articles, it looks like there are proposals to change the current system, but the council currently have no grounds to refuse the change. From what I can see, the property needs to be advertised for short let purposes for 140 days per year, which will be for this financial year. 

I contacted the council as soon as the property was advertised in early June 2022 and it still hasn't been rebranded and stopped the council tax payments. I have had a letter today saying they are waiting for the VOA, and unless I pay the council tax, they will take it to court. They have told me to continue the payments, and they will transfer the funds to the new account once it's opened. Due to the property size, I will be entitled to small business rate relief and am reluctant to pay them anymore as getting back will likely be a nightmare.
Attending court wouldn't phase me, but I wanted to ensure I was on sound legal footing before going down that route. 

I purchased the property back in March 2021, and development started to convert it immediately. My question is, at what point should the property have been graded for business use?
Many thanks