I’ve just bought “VA20 NITRILE Rubber V-Ring For Shafts 19-21mm” from a shop called Simply Bearings on E-bay UK, only cost £3.18 incl. postage, it stretches onto the controller knob and provides a lot more grip than the bare metal. I found that a number of retailers on the 'net do stock it also. I’ve no idea what it’s actually for, but it suits my purpose.