Take the tray out of the oven and put it on the hob, on a low-ish heat. Now you need to grab your masher and start squashing and grinding it all up. The tray will be hot and the hob will be hot so have a tea towel to hold the pan unless you have asbestos fingers. Make sure to scrape all the sticky bits from the bottom of the pan, the idea is to get things nice an carmalised. The longer you kind of fry it in the tray the darker the gravy will be. You should end up with a kind of vegetable and chicken-ey mash. Add the sherry or port if you want to and let it cook of for a few minute, its up to you. Get your flour and sprinkle it over and mix it in to the chicken-ey mass. Now pour in two litres of boiling water and stir in, Let it all simmer for a good half hour, stirring occasional to make sure it doesn’t stick letting it reduce and thicken.