Maybe they dress in livery? The Hogwarts elves dress in tea-towels with the Hogwarts crest. livery. And they must get the food from somewhere.

I don't ever remember seeing them come round the Strettons but that was before the Order controlled Dover. They can't be as picky now as they were.

I don't know if there's anyone in the Order who knows much about house elves. Hermione, maybe. (Because she knows things about everything, if there's a book about it.)