We present accurate quantum Monte Carlo calculations of the stability and structure of Mg+Hen and Ar+Hen clusters using accurate many-body potential surfaces derived from high level ab initio calculations and including spin-orbit effects in the case of Ar+. The highly quantum nature of the nuclear motion in these systems leads to significant delocalisation such that no sharp shell closure is found for Mg+ doped helium clusters whereas the interactions are sufficiently strong in Ar+ doped clusters to allow the appearance of magic numbers.