  Using a simple mean-field density functional theory theory (DFT), we
investigate the structure and phase behaviour of a model colloidal fluid
composed of particles interacting via a pair potential which has a hard core of
diameter $\sigma$, is attractive Yukawa at intermediate separations and
repulsive Yukawa at large separations. We analyse the form of the asymptotic
decay of the bulk fluid correlation functions, comparing results from our DFT
with those from the self consistent Ornstein-Zernike approximation (SCOZA). In
both theories we find rich crossover behaviour, whereby the ultimate decay of
correlation functions changes from monotonic to long-wavelength damped
oscillatory decay on crossing certain lines in the phase diagram, or sometimes
from oscillatory to oscillatory with a longer wavelength. For some choices of
potential parameters we find, within the DFT, a $\lambda$-line at which the
fluid becomes unstable with respect to periodic density fluctuations. SCOZA
fails to yield solutions for state points near such a $\lambda$-line. The
propensity to clustering of particles, which is reflected by the presence of a
long wavelength $\gg \sigma$, slowly decaying oscillatory pair correlation
function, and a structure factor that exhibits a very sharp maximum at small
but non zero wavenumbers, is enhanced in states near the $\lambda$-line. We
present density profiles for the planar liquid-gas interface and for fluids
adsorbed at a planar hard wall. The presence of a nearby $\lambda$-transition
gives rise to pronounced long-wavelength oscillations in the one-body densities
at both types of interface.
