Am not sure OP whether you are renting this property through one of the larger Holiday Cottage groups or privately.
Earlier this year I went through a similar situation with Sykes Cottages. Boris locked us down, Sykes refused to refund our Cottage costs.
Despite numerous avoiding tactics from Sykes, refusal to answer phones, ignoring emails etc, I battled on a number of fronts.
I involved my Credit Card company (despite having only paid a £60 booking fed through them) , made an Insurance claim (refused), complained to the CMA, complained on Trustpilot, etc.
I also contacted my Home Insurance company which gave me Legal cover and they said they would get involved if necessary.
It took me 3 months and 35 emails, including several to the CEO’s personal email address (found during a google search) before I got a wiff of success.
Of all avenues it was the CMA that came up trumps. They got involved and instructed Sykes to refund me. I got every penny back!!
Apart from the lesson that I will never again book a holiday through any of the Sykes stable of companies, I learned that it really can pay to be tenacious. Just because Sykes said no, it didn’t mean that I had to accept no for an answer.
Goid luck. I hope you can resolve your situation.