We study observational constraints on cosmological models with a quintessence field in the form of a dynamical pseudo Nambu–Goldstone boson. After reviewing the properties of the solutions, from a dynamical systems phase space analysis, we consider the constraints on parameter values imposed by luminosity distances from the 60 type Ia supernovae published by Perlmutter et al., and also from gravitational lensing statistics of distant quasars. In the case of the type Ia supernovae we explicitly allow for the possibility of evolution of the peak luminosities of the supernovae sources, using simple empirical models which have been recently discussed in the literature. We find weak evidence to suggest that the models with supernovae evolution fit the data better in the context of the quintessence models in question. If source evolution is a reality then the greatest challenge facing these models is the tension between current value of the expansion age, H0t0, and the fraction of the critical energy density, 0, corresponding to the scalar field. Nonetheless there are ranges of the free parameters which fit all available cosmological data.