Following on from the comments of @alison and @annette above re cancelling *unneccesary* appointments, part of the reason for my own lack of DNAs is that for about 30 years I avoided hospital diabetic clinics!
From diagnosis at age 10 to leaving home at 18 I was subject to the tedium and humiliation of my local hospital’s diabetic clinic.
However since then until about 18 months ago I had happily and effectively been taken care of by a succession of GPs who saw little point in referring me to a diabetic clinic since I seemed to know what I was doing and could ask sensible questions and often provide my own answers ( @seasiderdave – this definitely does not equate to a “lifetime of perfect control” 
)
However, my increasingly complex medical history (unrelated to the diabetes) had by then grown so convoluted that my GP had to admit that I needed more specialist help and referred me to the hospital endocrinologist (i.e. diabetic clinic).
The half dozen appointments I have had since then have been useful but apart from the first one have barely considered my diabetes.
The point I am getting to is that to reduce the number of DNAs that concerns your local group @tim, it seems obvious that either the outcome of each appointment needs to be valued by the patient (who should otherwise cancel it as Alison and Annette have done) or the appointment needs to be in a context that can provide that value (as with my GPs). The experience of @seasiderdave indicates how a lack of “value” can encourage DNAs. If I wasn’t getting value from my visits to the diabetes clinic in respect of my other issues, I don’t think I would continue to go just to get a HbA1c that I can get just as easily from my GP.