If possible cook in the pan in which the roast was cooked, if there is an excessive amount of oil remove this. Add the roasting pan bits and vegetable juices and bring to a boil. (I use the water from the vegetables I cook to serve with the roast, cabbage works well).
In a cup add the cold water, gravox and cornflour, mix so there are no lumps. While stirring pour this into the vegetable juices. Continue to stir until it thickens and starts to bubble again.
Once it has simmered for 3 minutes it is ready to serve, if too thick add some more water.