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