Setting up those clutches is a bit of a black art. In the 80s the roving Hayter Service man used to have an Osprey / 21 axle assembly set up on the bench in his high top van and ran an impromptu service course on site.
There should be a thick friction lining bonded to the large sprocket. I believe these are still available from an aftermarket supplier of friction materials.
However, from your image, it appears that there is a lot more to be done. Looking at the relationship between the bush and the axle it appears that one or the other - or both, are badly worn. I think that a complete strip and assessment of the axle is required.
https://www.dropbox.com/s/i5ys5rh1mdtrwc4/Hayter 21%2C Ospey Clutch adjustment0001.pdf?dl=0
https://www.dropbox.com/s/a2gbo2quuaymiea/Hayter Osprey and 21 0001.pdf?dl=0