Break up the heads of garlic so that all the cloves are separate, then cut off the hard root of each, but don't remove to much of the clove.
Put the cloves in the skins into a small saucepan of salted boiling water and bring back to a boil. Reduce the heat and simmer for 3-4 minutes, or until they are very soft, then drain.
Cut the potatoes in half lengthwise and put the watercress and salad leaves in a large bowl.
Heat the vegetable oil in a wok or large frying pan, add the bacon, and cook slowly for 10-15 minutes, or until it starts to crisp up.
Add the garlic (skins and all) and potatoes and cook until they are all golden brown.
Meanwhile, whisk the dressing ingredients together.
Pour the dressing over the salad and toss well. Pile equal amounts of the garlic, potato, and bacon mixture onto plates, then pile the salad leaves on top. Serve immediately.