Working on the basis that prevention is better than cure, one of the lightweight waterproof covers is a good idea, especially if parking under trees etc. They are advertised in Miscellany eg Walker St Clair or John Taylor for a larger one which covers the bonnet louvres as well.
I also find that the Mohair can get quite dusty (although not a problem in the current weather conditions!) and brushing with a clothes brush or something similar helps restore the appearance.
