This will be a moist, tender and succulent roast with a warm brown crust on it. You can serve it as part of a meal, and / or use leftovers for sandwiches, casseroles, etc. If you have a large enough roast to cook, you can let it cook overnight.
Start oven heating to 130 C / 250 F / Gas Mark 1/2. Adjust racks so that you will be able to place the roast on the lowest rack.
Mix the dry ingredients in a small bowl. Measure out the mustard.
In a roasting pan, place a wire rack. Rinse the roast with water, and pat dry with paper towel (discard these.)
Place roast upside down on the rack. With your hands, cover the bottom and sides with half the mustard, and sprinkle half of the dry mix on it; turn the roast over and repeat finishing off your ingredients.
Place roast in oven, pour a little water, wine or beer in roasting pan to cover bottom of pan. The purpose of the liquid is to help moderate the cooking temperature. (It won't keep the meat moist; that's a myth: only the fat in the meat will keep it moist.) If the liquid evaporates away while cooking, replenish.
Cook at this low temperature until meat thermometer next to bone (if there is one) reads 75 C / 170 F.
Towards the end, when the roast is approaching its done temperature, you can crank the oven up a bit if you need to speed it up. A 2 kilo (5 lb) roast will take about 5 hours; a 4 kilo roast (8 - 9 pounds) will take about 9 to 11 hours depending on your oven. So, roughly one hour per pound or 500g.
For the dry rub, the only real essential is brown sugar, salt and pepper. Top this up with whatever other dry spices or herbs you might have to hand.If you don't have garlic powder try a teaspoon of chopped garlic, or omit.For the mustard, use prepared mustard i.e. the kind in a jar. It doesn't matter whether you use a fancy French mustard, stronger English mustard, or good ole' neon yellow hot-dog mustard, all work equally well and give a nicely different taste.Serving idea: Pulled-Pork SandwichesIf you want to make "Pulled-Pork Sandwiches" from this roast, which are great for feeding a crowd, here are the directions: Scrape the drippings from the roasting pan into a saucepan, add 1 cup (8 oz / 250 ml) of water (if needed - you want to end up with about a cupful / 8 oz / 250 ml of sauce), and simmer till it thickens. Add if you wish, add anywhere from 1/2 to 1 cup (4 to 8 oz / 125 to 250 ml) of your favourite BBQ sauce, to taste. Meanwhile, using a fork and / or your fingers, pull the roast apart into small pieces (let the roast cool for an hour first). Mix the sauce and the pork in a larger pot and simmer till warm; serve on large buns to make sandwiches out of.If you are going to make this sauce, just use water in the bottom of the roasting pan as wine or beer will taste too strong and may even caramelize with a slightly burnt flavour.You can keep the sauce in the fridge, with or without the pork, about 3 days, so this is a good make-ahead dish for a crowd.Or you can mix the sauce with the pork and freeze till you need it.