  We have measured the transmission spectrum of the extremely inflated hot
Jupiter WASP-52b using simultaneous photometric observations in SDSS u', g' and
a filter centred on the sodium doublet (NaI) with the ULTRACAM instrument
mounted on the 4.2m William Herschel Telescope. We find that Rayleigh
scattering is not the dominant source of opacity within the planetary
atmosphere and find a transmission spectrum more consistent with
wavelength-independent opacity such as from clouds. We detect an in-transit
anomaly that we attribute to the presence of stellar activity and find that
this feature can be more simply modelled as a bright region on the stellar
surface akin to Solar faculae rather than spots. A spot model requires a
significantly larger planet/star radius ratio than that found in previous
studies. Our results highlight the precision that can be achieved by
ground-based photometry with errors in the scaled planetary radii of less than
one atmospheric scale height, comparable to HST observations.
