Neil,
The balm can work very well, but the leather needs to be dried out and absorbant so that it can soak in. A lot of time when aniline leather is damaged by the sun, the lacquer is still intact and only the dye has faded, meaning the balm cannot permeate the lacquer and so not colour the leather. This lacquer would then need to be removed so that the leather could be properly dyed again.