Slice the bacon, leaving the rind on, into 2.5 cm/ 1 inch lengths.
Fry until crisp, in a non-stick pan, in its own fat.
Wash and dry the trimmed watercress well; divide it on to four large plates.
Slice the warm or cold potatoes lengthways and arrange them on top of the watercress.
Spoon out the bacon and sprinkle it on to the potatoes.
Add the sunflower oil and the wine vinegar to the bacon fat, bring to the boil, scraping together the contents of the pan and spoon over the salads.