The study of spin-lattice relaxation in mononuclear lanthanide complexes is justified by the potential applications of these so-called single-molecule magnets in quantum computing and spintronic devices. We have designed a new family of tetragonal lanthanide single-molecule magnets. The precise symmetry of the crystal field is tunable, and consequently the compounds come in high and "low" symmetry versions. Initial muSR measurements on the higher symmetry holmium derivative on EMU at ISIS indicate that tunnelling dominates the zero-field relaxation. Further studies are however needed to confirm this and unravel the processes governing the in-field relaxation in lieu of tunneling. muSR measurements on the lower symmetry derivative will shed further light on the zero-field dynamics, as the symmetry-lowering is associated with a change in the ground state tunnelling probability.