Can excess salt in a cured gammon joint be removed after it has been roasted?

The best way to remove salt is by soaking it in non-salted water for several hours before cooking it. Theoretically, you could do the same thing after cooking but I would advise against it. Soaking cooked meat in water will likely result in an unpleasant waterlogged texture. I would instead suggest serving it with sides and sauces that are deliberately undersalted (or even cooked completely without salt). That way, the meal as a whole will even out and not be too salty.