Single-crystal derivatives of the spin - Peierls (S - P) system, , doped with Zn (0.5 to 2.4%) and Ni (1.7 to 6%) have been studied using SQUID magnetometry and neutron scattering. Our study shows that the impurities act to suppress the S - P state and produce a 3D Néel state at low temperatures. A phase diagram is constructed which shows that doping with either Zn (S = 0) or Ni (S = 1) leads to qualitatively similar results: the temperature at which the transition to the S - P state occurs decreases linearly with dopant concentration, whereas the Néel temperature, , initially increases to a maximum at around 4% and then decreases for higher concentrations. These results are discussed with reference to models of defects in 1D spin chains and to earlier experimental work on this system. One difference between the Zn- and Ni-doped samples is that in the Néel state the moments in the former point along the c-axis, while in the latter they are along the a-axis.