Cs3C60 is driven to a bulk superconductor without symmetry change by the application of pressure. Tc passes through a maximum of 38 K at 6 kbar. We have recently isolated the isostructural RbxCs3-xC60 materials with 0<x<3 in which the effect of physical pressure is mimicked by the application of chemical pressure through sovalent substitution of Cs+ by Rb+. We have initiated a research programme using TF-muSR in order to address the mechanism of the pairing interaction, study the gap symmetry and extract the magnitude of the superconducting gap in the fcc Cs3-xRbxC60 phases at ambient pressure as we cross the Mott insulator to metal/superconductor boundary and traverse the superconductivity dome. We will investigate the persistence of strong coupling well into the underexpanded regime and the very unconventional behaviour in the strongly correlated part of the electronic phase diagram.