Hi Jim here , I've had the same problem a few years ago with this shower mixer . This is a very weak point with this unit , as the splines on this little fellow are very fine and also it's made of plastic ! You can buy this piece on its own and easily change it in a few minutes ! This piece is for rotating the heat controller coil with in the mixer . Also make sure to put plenty of Vaseline on the assembly of this unit when replacing it . Ps , these spares are quiet expensive , and they are of only plastic , can't understand why they didn't make them from brass ! ..then we wouldn't have this problem accuring ever so often .
Jim Conlon, September 2015