Thanks for that, Pleiades - I suspected it was the knurled knobby and indeed began to twiddle it but found it extremely stiff and didn't want to force it. I may lube the thread overnight with WD40 to see if that makes it any easier.
Why oh why do m/cycle manufacturers now refer even the simplest of jobs to 'your authorised dealer'? In my old CB750 handbook all of the normal maintenance procedures are dealt with in detail....none of your 'refer to dealer' nonsense

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