Bring pot of potatoes to a boil then reduce heat and simmer for 10-15 minutes. Drain and cool under running cold water.
While potatoes cook, preheat oven to 220°C and cook bacon pieces. Remove once crispy and drain on kitchen paper.
In a large bowl, combine mayonnaise with mustard and lemon juice then season with salt and pepper. Quarter the cooked potatoes and toss with cos lettuce and half of the cherry tomatoes and bacon. Toss until well coated. Sprinkle remaining bacon and tomatoes over the top.