Bring the whole potatoes to the boil in a large saucepan with a good pinch of salt, and simmer for 10 mins - potatoes should be tender but not falling apart. Once cooked, drain and set aside to steam dry until cool enough (just) to handle - see notes.
Meanwhile, slice leek and roughly chop bacon.
Heat 1 tbsp of olive oil in a large frying pan over medium high heat, and fry bacon for 2 - 3 mins then remove to a small bowl (leaving fat in the pan) and set aside.
Turn the heat down to medium low. Add butter to pan, then leeks & fry for 4 - 5 minutes until leeks are soft. Depending on how much fat was left from your bacon, add the remaining olive oil to the pan if needed.
For the dressing, mix all ingredients in a small bowl to combine and set aside.
Once potatoes are cool enough to handle, chop each potato into 8.
When leeks are soft, add chopped potatoes to the pan along with the reserved bacon (plus any bacon fat!) and gently turn with a spatula for 1 -2 mins, then add dressing and continue to turn gently until dressing is well heated and incorporated.
Remove from heat, and add parsley to serve.