Snap off and discard any wilting or dark green outer lettuce leaves. Trim the leaves, then cut the head into quarters lengthways. Chop into chunks and get rid of the core. Wash under cold running water and dry in a salad spinner. To make the dressing, peel and finely chop the garlic, mix with the olive oil, and put in a bowl. Grate in the Parmesan and squeeze in the lemon juice. Add the Worcestershire sauce and yoghurt, and whisk together. Roughly chop and add the anchovies, season with a little sea salt and freshly ground black pepper, and whisk again. Toss the lettuce in the dressing, then scoop the salad into a serving bowl. Coarsely grate over a little extra Parmesan, if you like. Toss again, then serve.