Cancellation and refunds, measured only in notice
What is kept when a booking is called off depends on how much notice your message carries and on nothing else, and damages are never claimed on top of it.
Three lines cover almost every case. More than twenty-four hours before the agreed time, nothing is kept and the first half comes back whole. Inside the last day, while she is still at home, half of that first half is kept as fee already earned. Once she has set out, the first half is kept in full.
Two things that soften the last line
Where the hour you released is taken by another guest, the whole of the first half is returned, because in the end no time was lost. And where you can show the real loss was smaller than the share kept, only the smaller figure stays. Nothing beyond the share of the fee already earned is ever claimed; damages do not enter into it.
The booking with nothing behind it
A returning guest’s same-day hour or two at a city address carries no first half, so there is nothing for the scale to act on. Called off before she leaves, it leaves nothing owed. Called off once she is already travelling, it costs the part of the fee her journey stands for, and nothing further is asked.
Sending the message
One line in the thread is enough and no reason is needed. What matters is the hour it arrives, so send it when you know rather than when you have thought of how to phrase it. Where the evening has simply gone wrong rather than being unwanted, rescheduling is usually the better message, and the first move costs nothing. The worked figures, the busy dates and the distance-selling position all sit on the deposit and notice page.