Bring the water and stock cubes to a simmer in a large saucepan.
In a frypan, melt the butter and stir in the flour. Cook, stirring constantly, two minutes without browning.
Stir 1 tablespoon mustard into the stock and 1 tablespoon into the flour mix. Add some of the simmering stock to the flour while continuing to stir, then stir the flour back into the stock to thicken.
Add watercress and basil to the soup with salt and pepper to taste. Serve.