Single-Molecule Magnets (SMMs) are molecules that display slow relaxation of their magnetisation. We have designed a new class of mononuclear lanthanide SMMs, where the lanthanide ion is situated in a perfectly tetragonal crystal field. Depending on the lanthanide ion we have observed different magnetic relaxation processes, as probed by magnetic ac susceptibility. Here we intend to study two closely related holmium based systems, the only difference between the two being a change in point group symmetry at the lanthanide site. The low lying crystal field states measured with neutron spectroscopy and the lack of out-of-phase component in the magnetic ac susceptibility hints that the relaxation in these systems are so fast that muSR will be the ideal way to study the magnetic relaxation as a function of field and temperature in order to determine the nature of the relaxation processes.