For new potatoes I sometimes make a dressing of fat free natural yogurt with (sugar free) mint sauce stirred through to taste - it's good on cold chicken too.
If you can't manage without butter, you can still have it; just make sure you Syn it. Syns can be used to make meals tastier eg on ketchup, gravy, mayonnaise/salad cream - or, for you, a little bit of butter.