Preheat oven 200°C, Fan 180°C/ Gas 6.
Season the Welsh Lamb. Heat half the butter in a pan and sear the Welsh Lamb on all sides for five minutes until brown all over. Remove and set aside.
Heat the remaining butter and gently fry the mushrooms, chestnuts, garlic, thyme and cranberries for about 3-4 minutes. Add the port and allow to bubble for about a minute.
Place the mixture in a small bowl and leave to cool slightly. Save the pan to use for the sauce.
Cut the pastry block in half and roll into a large rectangle about 5mm thick (big enough to wrap around the Welsh Lamb). Spoon half the mushroom mixture down the centre of the rectangle and place the Welsh Lamb on top.
Brush the edges of the pastry with the beaten egg, fold in both ends followed by the sides wrapping the pastry around the meat.
Place on a baking tray, seam side down. Repeat with the second piece of Welsh Lamb. Brush the pastry all over with the remaining beaten egg and decorate with pastry trimmings if you wish. Bake in preheated oven for 20-25 minutes until pastry is golden brown.
Heat the pan used for cooking the mushroom mixture, add the port & swirl around pan to get all the juices etc into the mixture. Add the redcurrant jelly and simmer for about 10 minutes to reduce slightly. Remove the wellingtons from the oven and leave to rest for 5-10 minutes before carving into thick slices.
Serve with port sauce and seasonal steamed vegetables or red cabbage braised with apples & red wine.