  We demonstrate a numerical technique that can evaluate the core-to-core
variations in propagation constant in multicore fibre. Using a Markov Chain
Monte Carlo process, we replicate the interference patterns of light that has
coupled between the cores during propagation. We describe the algorithm and
verify its operation by successfully reconstructing target propagation
constants in a fictional fibre. Then we carry out a reconstruction of the
propagation constants in a real fibre containing 37 single-mode cores. We find
that the range of fractional propagation constant variation across the cores is
approximately $\pm2 \times 10^{-5}$.
