  We present new K- and L'-band imaging observations for members of the young
(3-5 Myr) sigma Orionis cluster, obtained with UIST at UKIRT. We determine
(K-L') colour excesses with respect to the photospheres, finding evidence for
warm circumstellar dust around 27 out of 83 cluster members that have masses
between 0.04 Msun and 1.0 Msun. This indicates a circumstellar disc frequency
of at least (33 +/- 6)% for this cluster, consistent with previous
determinations from smaller samples (Oliveira et al. 2004) and also consistent
with the 3 Myr disc half-life suggested by Haisch et al. (2001). There is
marginal evidence that the disc frequency declines towards lower masses, but
the data are also consistent with no mass-dependence at all. There is no
evidence for spatial segregation of objects with and without circumstellar
discs.
