The analytical models used in this work replicate those outlined in the work of Campbell et al. , pictorial descriptions of which can be found in Figure . These particular models were chosen for there relevance to available data and recent publication, however, we expect that the specifics of the analytical models would have little impact on the results of this work. These were implemented in the refnx reflectometry analysis package and the models can be found in the electronic supplementary information for this publication .