The complex structure of gamma-ray burst (GRB) afterglow light curves revealed by Swift has made their interpretation  more difficult
than in the pre-Swift era. Indeed many of the pre-Swift models of GRBs have been thrown into doubt or have had to be modified. Even the
favoured model -- a blast wave due to the shock front of a collimated ultra-relativistic jet ploughing into the circumburst medium -- has
been called into question. We aim to test the validity of this blast wave model and to constrain its characteristic physical parameters
by comparing the observed multi-wavelength light curves and optical to X-ray spectral energy distributions of our sample to the
predictions of the blast wave model.

We can successfully interpret the majority of the bursts in our sample within the framework of the blast wave model and we estimate the
electron energy distribution index, p, for these bursts as well as a larger, statistically significant sample. Our analysis of the
distribution of p reveals that, even in the most conservative case of least scatter, the values are not consistent with a single,
universal value as suggested by some analytical calculations or simulations. However, neither is the width of the distribution as wide as
suggested by other authors. Given that this acceleration process applies to many astronomical jets, such as those from X-ray binaries and
AGN, the accurate measurement of the distribution of p is of fundamental importance to differentiate between the possible theories of
electron acceleration at any relativistic shock front.