Pour most of the oil from the potato roasting tin into a saucepan about 5 or 10 mins before they've finished cooking (I usually turn the oven up at this point make sure the pots are nice and crisp). Heat oil gently for a couple of mins, take of the heat and add 1 or 2 tablespoons (depending on how much gravy you need) of flour and heat gently for few minutes. Take of the heat and stir in water from your boiled veg, return to the heat and bring back to the boil, add yeast extract (marmite) and soya sauce (and fresh herbs - especially marjoram or savory) to taste. Simmer until thickened. If it doesn't thicken mix some flour with more vegetable water in a bowl and add to gravy until thickened. Prepare to taste one of the best vegetarian dinners you could wish to eat. Oh, okay then, add a slice of roast meat if you wish ;>)