Lobster thermidor baked potatoes

Name: Lobster thermidor baked potatoes
Yield: Serves 4
Ingredients: 1 x 500-600 g cooked lobster; 1 small egg yolk; Salt and freshly ground black pepper; 50 ml white wine; 1 large shallot, peeled and finely chopped; 2 tsp English mustard; 4 baking potatoes; 80 g butter; 100 ml fish stock (or a third of a good-quality stock cube dissolved in 100ml hot water); 300 ml double cream; 30 g parmesan, grated; 80 g mature cheddar, grated