  We present experimental results on resonant spin tunnelling in a single
crystal of Mn$_{12}$-2Cl benzoate with different concentration of dislocations.
The time evolution of the magnetisation follows the stretched exponential over
a few time decades. The values of parameters of stretched exponential deduced
from experiment have been used to determine the concentration of dislocations
before and after the cooling-annealing process, using the algorithm recently
suggested by Garanin and Chudnovsky.
