It sounds like part of the knob collar has broken off and is still on the shaft .
Get new knob:
http://www.ebay.co.uk/bhp/new-world-cooker-knobs (Care - make sure you get the exact right part)
http://www.partmaster.co.uk/newworld/cooker-oven/knob/catalogue.pl?shop=newworld&path=263566&refine=knob
pull off the old bit and push on new one.
Acp
August 2017
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