Frankfurter and Potato Salad

This is my version of a salad that Harrods used to sell in their food hall.

1 1/2 tbsps olive oil
1/2 tbsp wine vinegar
pinch dry mustard powder
1/4 tsp salt
freshly milled black pepper
small potatoes with nice skins
1 package of frankfurters
1 small onion finely chopped
2 gherkins, finely chopped

Boil the potatoes in their skins until cooked, but firm. Cool, cut into quarters and set aside. Slice the frankfurters diagonally. Put in a serving bowl with the chopped onion. Whisk together the remaining ingredients to make the salad dressing. Pour over the salad and toss thoroughly.