away from the cluster centre and data where it lies close to a reference arc are flagged out (Section \[sec:corr\]). Where no 28.5GHz flux density is available, we proceed with the GBT data alone. Such cases are unlikely to be crucial, as the non detection by OVRO/BIMA suggests the source is either rather faint, or well away from the central regions of the cluster. Many sources have GBT data which are usable but rather noisy. In such cases, the 28.5GHz data are invaluable in tying down the 30GHz flux density due to their comparatively small uncertainties.

Seven sources have no usable GBT data: two in A1835, one in A2219, one in A1763, one in A68 and two in A773. We now discuss our method for each source in turn. Source 1 in A1835 was measured twice by OVRO/BIMA, and also appears in NVSS. We fit a power law to the three measurements. As our observing frequency is close to that of OVRO/BIMA, assuming a constant spectral index is unlikely to have a large impact. We rule out significant variability. Source 2 in A1835 was also measured twice at 28.5GHz, but does not appear in NVSS. We consult FIRST for a 