Purée the tomatoes, cucumber, pepper, spring onion and vinegar in a food processor or with a hand-held mixer. Transfer to a large bowl.
Stir in the watercress (do not purée it or it will become sloppy). Season to taste.
Place 3 tablespoons of water in a teacup, sprinkle the gelatine over the surface and leave it for 5 minutes until it becomes spongy. Then stand the cup in a pan of simmering water until the gelatine is runny and clear. Pour it into the tomato mixture in a steady stream, stirring to combine it thoroughly.
Grease four 200 ml (7 fl oz) ramekins. Spoon in the mixture and chill for 1 hour, or until set.
Turn out the moulds by dipping the bases in hot water then inverting them over a plate. Garnish with watercress and serve with brown bread.