radio galaxy PKS B1138-262. Similar results have been achieved for the radio galaxies TN J1338-1942 [$z=4.1$; @Venemans02], TN J0924-2201 [$z=5.2$; @Venemans04; @Overzier06] and MRC B0316-257 [$z=3.13$; @Venemans05] and 6C0140+326 [$z=4.413$; @Kuiper11].

While this combination of broad and narrowband imaging has produced demonstrably successful results, the more direct antecedents of this work have adopted an alternative approach. The *Taurus Tunable Filter* (TTF) instrument, installed on the Anglo-Australian Telescope, provided a powerful method of narrow-band (of order 10 Å) imaging over a large range of wavelengths [@BH982]. @Bremer99 introduced the strategy used to search for line emitters at a given redshift with TTF: broadly, the tunable filter is stepped across a range of wavelengths around the expected redshifted position of the emission. Emission line galaxies then appear brighter in those frames centred on the spectral line.

Considerable success has been achieved at lower redshifts with this technique. @Baker01 located a cluster around the $z = 0.9$ radio-loud quasar MRC B0450-221 using TTF to search for $[$O[ii]{}$]$ 3727 Å emission. The same technique was used by @Barr04, who examined six radio-loud quasars at redshifts $0.8 < z < 1.3$, identifying a total of 47 candidate emission line galaxies (ELGs), at an average space 