Heat the grill. Put the ribs in the grill pan and brown them under the grill, turning frequently, to seal in the juices. If you don't have a grill, brown the ribs in a frying pan, with a little oil or butter, over a medium heat, for 2-3 minutes, turning often. Peel and crush the garlic clove.
Mix the soy sauce, marmalade and garlic, and spread over the ribs. Peel and slice the onion. Put the onion in a casserole or ovenproof dish. Place the ribs on top and season with salt and pepper.
Dissolve the stock cube in 1/2 cup of boiling water, add the vinegar, and pour it all over the ribs. Cover and cook in a hot oven (400F/200C/Ga Mark 6-7) for 1 1/4-1 1/2 hours (the longer time for the larger amount). Remove the lid for the last 20 minutes to allow the meat to become crisp. The sauce should be sticky when cooked. The jacket potato can be cooked in the oven with the casserole.