The flow control knob for my shower has worn out. Inside it has a hard plastic insert which slides onto a brass knurled thing...OK, I have no idea how to describe it - it's about 1cm long and has a circular cross section looking like a gear out of a watch. The plastic bit used to marry up nicely with the brass bit and turn it to open/close the water flow. Now it's worn smooth and the brass bit stays put when I turn the knob.
Any ideas how to mend this or find a replacement without buying a whole new fitting? I have no idea of the manufacturer (it was fitted by previous owners) and it doesn't look like my favourite solution (a blob of epoxy resin) would do the job.
TIA
John