small fraction of the error on the mean were seen. The distributions of data values in the double-difference data are close to Gaussian, with a slight tendency to show elevated wings in the distributions: the estimates of the error on the mean (Table \[tab:res\]) take account of the full distribution, and not the distribution after truncation of outliers.

RADIO SOURCE CONTAMINATION {#sec:src}
==========================

Radio source contamination remains a significant problem for observations of the Sunyaev Zel’dovich effect. If we know the positions and 30GHz flux densities of all sources near our observing locations, we can correct the data and establish the true magnitude of the SZ effect for each cluster. Ideally, we would consult a survey of the radio sky at a frequency close to 30GHz in order to identify contaminant radio sources. The only such survey currently available is the WMAP point source catalogue ([@Wright2009]), but this is complete only to the 2Jy level and so is three orders of magnitude too shallow for our purposes. We consult the all–sky NVSS ([@NVSS], 1.4GHz) and GB6 ([@GB6], 4.85GHz) catalogues and retain all sources within $5'$ of the pointing centre for each cluster. In addition, we include any sources found in 