Using a small, sharp knife, score a zigzag pattern in the skin around the top of each pear, working downwards from the stalk. Leaving the zigzag skin at the top in place, peel the rest of each pear very thinly so as to preserve their shape.
Remove the core from the base using a corer or small melon baller. Cut the orange zest into thin strips.
Combine all the ingredients in a saucepan big enough to hold everything, including the pears. Set over a medium heat until the liquid comes to the boil, then reduce the heat so that it is barely simmering. The cooking time will vary according to the ripeness of the pears. Very ripe pears will only take 5 minutes, much less ripe fruit will need up to 15 minutes. As soon as they are ready, turn off the heat.
Transfer the pears with their poaching syrup to a bowl and leave at room temperature until needed. Serve one or two pears on each plate with a little orange zest and some of the syrup.