The term "holiday let" doesn't refer to the entire length of the Summer break. It just means a place where people stay when they're on holiday, for a few days or whatever. They don't sign leases.
She does state on the website that she's prepared to do longer-term lets (which doesn't bother us because, as you say, they sign a lease and have to provide references), but what she wants to use it for is short stays of a couple of days.
It's already reserved for 5 days over Hogmanay/New Year.
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