Hey-up Nosha
There isn't a lot that you can do with cooked lamb. However the original recipe for Shepherd's Pie was for lamb, hence the name. Have you got a mincer or food processor? Simply cut all the meat from the bone and put it through the mincer or processor. Chuck it in a pan with some sweated onions (that means lightly fried).
Dissolve a couple of Oxo or Bovril cubes (or Bisto if you prefer a thicker gravy) in about a pint of boiling water, with a couple of squirts of tomato puree or ketchup, and add it to the pan. Bring back to the boil and simmer for a few minutes.
Peel and boil some spuds and mash them. with butter, milk, and sage.
Strain the meat, and put it into a pie dish and cover with the mash. Garnish with a few slices of tomato and put it in the oven (about 150 degrees C or mark 3 or 4) for about half an hour, or until the top is brown, and serve with the fravy and veg of choice.
Enjoy!