Bobbin casing issues with Singer 449?

I was happily sewing along, when my needle snapped for no good reason. It appears that the metal ring round the bobbin case that turns as you sew does not turn any more. It can be helped along and will make a revolution or so before getting stuck again. I have cleaned out a lot of lint and have opened it up, but I can't see where might need oiling/cleaning specifically. The belt looks ok; I think the problem is more with the metal turny bit rather than the belt but I may wwll be wrong.

Thanks in advance