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This. It's not in the tenancy agreement it's in your contract with the agency. We're in a similar position but with winkworth not KFH can't remember off the top of my head the exact amount but it is considerably less than if we went with them to put the flat on the market. Something like 0.7% Vs 1.5% but don't quote me on that.
If you wanted to be really shitty and knew that your tenants would follow through you could cancel your agreement with KFH and rent it privately to your tenants for X months then agree a private sale but there is considerable risk there from both sides.
There wouldn't be anything in the tenancy agreement but I'd be very surprised if an agent like KFH doesn't have it covered in their terms of business.