Hand wash with a little wool wash and warm water, gently squeezing the solution through the cover.
Let soak for 15-30min, then rinse in clean water until it runs clear.
Next step is to lanolise: melt a little lanolin in hot water, dilute this with cool water to make a warm lanolin soaking solution. Squeeze through gently and soak your cover in it for at least 30min, then remove excess water by rolling in a towel and dry flat in the shade.