EDIT: previously one star review for very late cancelled operation and only solution was a six-month further wait for cat castration, meaning six months of an indoor cat and possible behavioural changes.
However, within a couple of hours after formally complaining, they had rearranged to go to another vet practice in their group at exactly the same time/day.
At both the initial call and subsequent call they were very apologetic and kind - mistakes happen with booking etc, but you can only be judged on how you deal with it and they sorted very quickly!!