I've just finished a PPL at Cumbernauld Flying Centre (waiting on the CAA sending everything back).
I flew their DA20 Katanas and the Robin HR200, mostly with the same instructor.
I think it's easier to keep the same instructor if you book well in advance; they also have more people available at the weekend than on weekdays.
No complaints with the organisation, no lessons were cancelled due to aircraft trouble, although plenty due to the Cumbernauld weather.
Last I heard, there were a couple of Katanas waiting on new engines being sent from the manufacturer; again, booking in advance meant I was also still able to fly regularly.