Gritty transmission system on Brompton

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Something in my Brompton's transmission system is feeling rather gritty and clunky, but I can't work out what it is. I was suspecting it might be a pedal, but now I'm wondering if it's the hub. Any ideas?
 

Rohloff_Brompton_Rider

Formerly just_fixed
hub adjustment...the little chain.
 
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