I used to fly the Slingsby, and went through a pretty comprehensive spin training exercise before flying solo aerobatics. I have also spoken to two very experienced Slingsby instructors (who teach spin recovery in the T67 almost on a daily basis) and both stated that if you apply the correct spin recovery controls and keep them applied - it WILL stop. Every time.
If you don't, it won't.
So, the moral seems to be, allow plenty of height for your practice aeros and get the proper training.