Halve the lemons, quarter the Jersey Royals, then pick and finely chop the tarragon and parsley.
Put the chicken, unpeeled garlic cloves, lemon and potatoes in a large baking tray, drizzle with oil, season, then toss together and pop in the oven for 45 minutes, or until golden, crispy and cooked. Remove from the oven and allow to cool.
Meanwhile, melt the butter in a large non-stick saucepan until bubbling. Add the flour and stir for 1 to 2 minutes to form a paste, then slowly pour in the stock, whisking until thickened. Add the mustard, then season.
Once the chicken is cool enough to handle, remove and reserve the skin and discard any gristle. Shred the meat, discarding the bones.
Squeeze the garlic from its skins back into the roasting tray and squeeze over all the juice from the lemons and toss well.
Pour over the thickened stock and add the herbs and watercress. Check the seasoning, then finely chop the chicken skins and sprinkle over the filling.
Lay out sheets of filo on a clean surface and cut each one in half. Lightly scrunch up and place on top of the pie filling, brushing with the melted butter.
Bake for 25 minutes, or until crispy and golden, then serve with a crisp green salad.