T350 grooved / serrated nut the washer arm sits on

[NWTony]

OK, I can't describe it much better than in the title, but its the cone shaped, star / sort of torx shaped grooved nut the bottom of the wiper arm sits on, presumably so you can adjust the start stop position a little. Mine has apparently worn smooth over the years so I'm looking to replcae it before the wiper blade catches on the windscreen edge and snaps (again). Does anyone have any idea what it's real name is and more importantly, where can I get one from?

Thanks.