We have analysed the strength of the UV upturn in red sequence galaxies with luminosities reaching to below the L ^ { * } point within four clusters at z = 0.3 , 0.55 & 0.7 .
We find that the incidence and strength of the upturn remains constant up to z = 0.55 .
In comparison , the prevalence and strength of the UV upturn is significantly diminished in the z = 0.7 cluster , implying that the stellar population responsible for the upturn in a typical red sequence galaxy is only just developing at this redshift and is essentially fully-developed by \sim 1 Gyr later .
Of all the mainstream models that seek to explain the UV upturn phenomenon , it is those that generate the upturn through the presence of a Helium-enhanced stellar subpopulation on the ( hot ) horizontal branch that are most consistent with this behaviour .
The epoch ( z = 0.7 ) where the stars responsible for the upturn first evolve from the red giant branch places constraints on their age and chemical abundances .
By comparing our results with the prediction made by the YEPS Helium-enhanced spectrophotometic models , we find that a solar metallicity sub-population that displays a consistent upturn between 0 < z < 0.55 but then fades by z = 0.7 would require a Helium abundance of Y \geqslant 0.45 , if formed at z _ { f } \sim 4 .
Later formation redshifts and/or higher metallicity would further increase the Helium enhancement required to produce the observed upturn in these clusters and vice versa .