The bearing cover has to be prised away with a screwdriver or chisel, and underneath you'll find a large hexagon nut that must be undone. Its staked into position but it will wind off anyway (normal RH thread). The drum will then theoretically come off but you may need a puller if its been on a while. I'd bet that the brake shoe lining material has come away, causing the jam - done loads of these, and not just Citroens! Cheers.
John, January 2008