  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 $Y\geqslant0.45$, if formed at $z_f\sim4$. 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.
