I've sorted it now, kind of. It feels loads better, not 100% like new but I'm happy anyway! I bled the brake with oil, let it sit for an hour, blew out with pump, bled through 3-4 times with water, then I replaced one of the slaves, which was the real troublemaker. My brake now has a silver lever, one silver slave, and one red slave. Looks horrible but feels much better!