redshifts [e.g. @Kodama2007; @Stanford2012; @Newman2014].

In Figure \[fig:CMD\] we show the $i'-J$ colour-magnitude diagram of sources within 0.9arcmin of the RLAGN ($\sim500$kpc at this redshift)[^6]. Larger squares indicate sources that are within 30arcsec of the radio galaxy. The histogram in the top panel of Figure \[fig:CMD\] shows the excess number of galaxies in 7C, compared to the UDS control field. There is a significant overdensity in the field around C at all magnitudes, increasing at the faint end. Although we expect contamination from fore- and background field sources in the colour-magnitude diagram, the majority of the red data points are likely to be cluster members, and there is a clear, strong red sequence at $J<23$mag, with hints of the sequence continuing to fainter magnitudes.

The red sequence is fit by the line $i'-J = 7.688 - 0.232 \times J$, calculated by iteratively clipping sources more than 1.5$\sigma$ from the best-fit line, allowing both the slope and normalization to freely vary, until convergence was reached. The fit is shown by the red dotted line in Figure \[fig:CMD\]. The colours of sources on this red sequence suggest they formed at redshifts of $2<z_f<3$, assuming the galaxies formed their stars in single bursts. 