  We use galaxies from the Herschel-ATLAS survey, and a suite of ancillary
simulations based on an isothermal dust model, to study our ability to
determine the effective dust temperature, luminosity and emissivity index of
250um selected galaxies in the local Universe (z < 0.5). As well as simple
far-infrared SED fitting of individual galaxies based on chi^2 minimisation, we
attempt to derive the best global isothermal properties of 13,826 galaxies with
reliable optical counterparts and spectroscopic redshifts. Using our
simulations, we highlight the fact that applying traditional SED fitting
techniques to noisy observational data in the Herschel Space Observatory bands
introduces artificial anti-correlation between derived values of dust
temperature and emissivity index. This is true even for galaxies with the most
robust detections in our sample, making the results hard to interpret. We apply
a method to determine the best-fit global values of isothermal effective
temperature and emissivity index for z < 0.5 galaxies in H-ATLAS, deriving T =
22.3 +/- 0.1K and Beta = 1.98 +/- 0.02 (or T = 23.5 +/- 0.1K and Beta = 1.82
+/- 0.02 if we attempt to correct for bias by assuming that T and Beta are
independent and normally distributed). We use our technique to test for an
evolving emissivity index, finding only weak evidence. The median dust
luminosity of our sample is log(Ldust/Lsolar) = 10.72 +/- 0.05 which (unlike T)
shows little dependence on the choice of Beta used in our analysis, including
whether it is variable or fixed. We use a further suite of simulations to
emphasise the importance of the H-ATLAS PACS data for deriving dust
temperatures at these redshifts, even though they are less sensitive than the
SPIRE data. The majority of galaxies detected by H-ATLAS are normal
star-forming galaxies, though a substantial minority (~31%) fall in the
Luminous Infrared Galaxy category. (Abridged)
