You can move a visit yourself from your account up until the full-charge window opens. Inside the free-change window there is no fee at all — we simply re-quote the visit at the new time, so an evening or holiday rate may be added or removed and the difference is charged or refunded automatically.
After the free window closes, the late-change fee for your tier (50% of the visit) is kept and the visit is re-booked at the new time. Once a visit is inside its full-charge window it can no longer be moved online — message us and we'll do what we can.