26.0$ ($5\sigma$, point source, 2”) over an area of 0.8 deg$^2$. The size of the UDS field significantly reduces the effects of cosmic variance and on this scale is the deepest near-infrared survey to date. For this work we use the UDS DR1 release [@Warren_etal:2007], which reaches $5\sigma$ (point source) depths of K$_{AB} = 23.5$ and J$_{AB} = 23.7$. For details of the completeness estimation, image stacking, mosaicing and catalogue extraction procedures see [@Foucaud_etal:2007] and Almaini et al. (in preparation). In addition to these data, deep $B,V,R,i^{\prime},z^{\prime}$ imaging is also available from Subaru Suprimecam to limiting depths of $ B_{AB} = 28.4, ~V_{AB} = 27.8, ~R_{AB} = 27.7,
~i_{AB}^{\prime} = 27.7$ and $ z_{AB}^{\prime} = 26.7$ [@Furusawa_etal:2008]. The UDS and Subaru survey areas are not entirely coincident, which reduces our usable area to 0.63 deg$^2$ in this analysis.

Construction of the BzK colour-colour diagram
---------------------------------------------

The original BzK selection technique was introduced in @Daddi_etal:2004 based on photometry in the Bessel $B$-band, Gunn $z$-band and $K_s$-bands as defined on the FORS1, FORS2 and ISAAC instruments at the Very Large Telescope (VLT). To correct our colours to these filters we used the stellar track of K06, who in turn used published empirical 