Nickel based superalloys are extensively used in the aero-engine and power generation industries for high temperature regions of gas turbines.  There is a drive to increase the operation temperature of engines in order to improve fuel efficiency. This has lead to the development of advanced polycrystalline disc alloys such as RR1000 with a γ′ volume fraction of nearly 50%. There has been a significant amount of work on low volume fraction Ni-base superalloys, however, the way in which the deformation mechanism changes with increased volume fraction is not well understood. Conventional microstructures of such alloys exhibit a bi- or tri-modal γ′ distribution making mechanistic studies very difficult. For this reason, model microstructures with simplified uni-modal γ′ distributions (80 nm, 120 nm and 250 nm) have been developed for RR1000. In-situ loading experiments using neutron diffraction were performed to investigate the elastic strain response of γ and γ′ during plastic deformation at room temperature 500°C and 750°C. The results show that the level of load partitioning between γ and γ′ generally increases with increasing particle size and higher test temperature. A two-site EPSC model has been employed to identify possible mechanisms for such changes in load transfer. Results indicate that the observed changes of load transfer can be understood in terms of changes of the γ′ slip mode from {111} to {100} with increasing temperature and an increase of critical resolved shear stress in γ′ with increasing particle size. The plasticity modelling results are supported by detailed post-mortem electron microscopy studies showing less particle shearing with increased particle size and test temperature.