Place the joint (roast) in a roasting bag with the water, pierce the bag and close it with a non metallic tie. Stand it in a dish and cook for 6 minutes on high. Reduce to medium and cook for a further 25 minutes, turning over after 12 minutes.
Remove the joint from the bag and wrap it loosely in foil, shiny side inwards. Let it stand for 15 minutes.
Place the pineapple juice, sugar and butter in a bowl. Cook on high for 6 minutes, stirring every 2 minutes, to make a glaze.
Stand the joint in a grill (broiling) pan, brush with the glaze and brown it under a preheated conventional grill (broiler).