The extreme quantum nature of muons and muonium in condensed matter is the result of having a mass that is nine times lighter than that of a hydrogen atom. While this quantum nature may often be masked by thermal excitations, at low temperatures its effects can become more apparent. This was clearly demonstrated in solid benzene ALC spectra below 120 K, where both intramolecular and intermolecular multisite quantum tunnelling of radical states was observed. Selective site substitution of the benzene molecule can control the sites available for quantum tunnelling and this has been demonstrated clearly for p-xylene in the previous beam time. We now propose to explore three further examples of methyl substituted benzenes, o-xylene, m-xylene and durene in order to carefully investigate the dependence of the muonium tunnelling on the details of the crystalline environment.