ISIS operates an 800 MeV Rapid Cycling Synchrotron (RCS) delivering protons to neutron and muon targets with a beam power of 0.2 MW. A reliable lattice description is essential for advancing low-loss, high-intensity operation and supports emerging model-driven optimisation enabled by forthcoming Python-accessible controls. A new consolidated low-intensity linear optics model of the ISIS RCS, developed through a continuous programme of systematic measurement-based benchmarking, is presented. Using enhanced analysis of low-intensity turn-by-turn BPM data, multiple optics measurements have been integrated into a single self-consistent lattice representation. This lattice represents a significant advance over earlier design-only or partially benchmarked descriptions, enabling clearer identification of optics discrepancies and guiding targeted correction measures, including improvements to magnet survey and alignment. The resulting reference lattice, implemented in MAD-X/cpymad and cross-checked with PTC-PyORBIT, reproduces low-intensity optics with improved predictive performance, particularly for orbit response and optics control. Benchmarks of the consolidated model, resulting improvements, and planned implementation in model-based optimisation of ISIS RCS operation are presented.

The ISIS Neutron and Muon source produces pulsed neutrons and muons for multi-disciplinary scattering and spectrometry experiments. Up to 3e13 protons per pulse are accumulated over a ~200 µs injection period, then accelerated from 70 to 800 MeV in the 50 Hz rapid cycling synchrotron. This beam is then extracted to one of two spallation neutron targets. An electrostatic chopper is installed in the transfer line between the injector linac and the synchrotron, which allows injection of beam pulses between 100 ns - 5 µs, filling a fraction of the machine acceptance of the synchrotron. Beam Position Monitor (BPM) measurements of chopped beams are routinely used to determine synchrotron lattice parameters. As part of ongoing improvements to measurement, modelling and control of ISIS beam dynamics, a new tool has been developed to routinely use these BPM signals to generate phase space ellipses. Preliminary results from commissioning of this tool are presented, showing good agreement with the linear transverse model of the ISIS RCS.