If it was on the market last year, probably before you booked, then I think you've been 'had over'.
Honestly, is one supposed to check such things for oneself on Zoopla when you book?
Terms and conditions for holiday lets seem to be written for the advantage and protection of the owner and letting agent rather than the customer. No sense myself of how reasonable that is in terms of customer protection legislation.
My inclination caught thus, assuming that the agent hasn't anything on their books nearby and to which you could transfer at no cost/minimal inconvenience, would be to threaten agent and or landlord with proceedings for the extra costs. Or compensation for loss of a holiday and out of pocket spends - eg pre booked trips.
www.citizensadvice.org.uk/consumer/
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Holiday Cottage Cancellation 
Holiday Cottage Cancellation 