I had made an appointment through Treatwell for a particular time & had paid in advance (I think that was the only option), however I received a call the subsequent day saying that, that slot was not available as someone had booked through their system. Further they added that those who book through their system would be given priority for the slot, Anyway I managed to rescheduled my other commitments and make at the time a slot was available.
In parlour experience was great.