I ordered some new parts for the clutch, borrowed a special puller for the clutch and bought a new impact gun. This meant that I could undo the crank nut without having to lock the crankshaft.
Primary cover removed, then crank nut and dynamo nut removed.
Dynamo belt and pulleys removed.
So, I thoroughly cleaned the clutch and fitted new springs but can't re-fit it until I find a way to remove the sprocket.