Illustration of a mock spectrum with the U-band (LyC) and V606 (non-ionising UV) filters marked by the shaded areas. The Lyman limit is indicated with the vertical dashed black line. The fundamental observable, the LyC–to–non-ionising–UV flux ratio, ℛ_obs (see Sect. 2.2) measures the flux captured by the U-band (left) divided by the flux captured by the V606 filter (right). To ensure that the U-filter is always capturing flux short-ward of the Lyman limit, and is thus measuring ionising photons, we imposed a lower redshift bound of z = 3.35 on the galaxy sample. This mock spectrum was obtained from the intrinsic BPASS spectrum that was fitted to the stacked VANDELS spectrum in Begley et al. (2022) (see Sect. 3.2). The flux below the Lyman limit was attenuated according to an assumed f_esc = 20%, whilst dust attenuation was applied to the non-ionising region using the Reddy et al. (2016b) dust curve and a value of A_UV = 2.3. The full spectrum was multiplied with a simulated IGM transmission (exhibiting a transmitted fraction of 14% through the U filter; see Sect. 3.3).