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Using GALEX, UVOT and optical photometry, we explore the prevalence andstrength of the UV-upturn in the spectra of quiescent early type galaxies inseveral nearby clusters. Even for galaxies with completely passive opticalcolours, there is a large spread in vacuum UV colour consistent with almost allhaving some UV upturn component. Combining GALEX and UVOT data below 3000\AA,we generate for the first time comparatively detailed UV SEDs for Coma clustergalaxies. Fitting the UV upturn component with a blackbody, twenty six of theseshow a range of characteristic temperatures (10000--21000K) for the UV upturnpopulation. Assuming a single temperature to explain GALEX-optical colourscould underestimate the fraction of galaxies with UV upturns and mis-classifysome as systems with residual star formation. The UV upturn phenomenon is notan exclusive feature found only in giant galaxies; we identify galaxies withsimilar (or even bluer) $FUV-V$ colours to the giants with upturns over a rangeof fainter luminosities. The temperature and strength of the UV upturn arecorrelated with galaxy mass. Under the plausible hypothesis that the sources ofthe UV upturn are blue horizontal branch stars, the most likely mechanism forthis is the presence of a substantial (between 4\% and 20\%) Helium rich ($Y >0.3$) population of stars in these galaxies, potentially formed at $z\sim 4$and certainly at $z>2$; this plausibly sets a lower limit of $\sim {\rm 0.3 -0.8} \times 10^{10}$ $M_{\odot}$ to the {\it in situ} stellar mass of $\simL^*$ galaxies at this redshift.

UV SEDs of early-type cluster galaxies: A new look at the UV upturn