Preheat the oven to 170°C/Gas Mark 3. Line a large roasting tin with baking paper.
Place the pork joint in the prepared tin and, if any string has been used to tie it together, remove. Score the skin, then drizzle over some olive oil and rub it into the meat.
Mix the oregano, celery salt, mustard powder, pepper and coriander together in a small bowl. Scatter the mixture all over the pork and use your hands to rub it in, especially on the underside and deep into the skin on top. Pour the beer around the pork and add a little splash on top, ensuring you don’t rinse off the spice mixture. Roast for four hours, or until cooked through – check the joint after three hours and if it is browning excessively, cover it with a double layer of kitchen foil for the remaining cooking time.
If the skin has not crisped up when the cooking time has elapsed, remove the pork from the oven and increase the oven temperature to 220°C/Gas Mark 7. Using tongs, carefully peel the skin off the meat and lay it flat on a rack over a baking tray. Roast the skin for a further 10-15 minutes, or until crispy. Meanwhile, cover the pork loosely with foil and leave to rest for about 30 minutes.
To make the sauce, put the plums, ground coriander and cayenne pepper into a saucepan set over a medium-low heat and season with salt and pepper. Sweat the plums for about 15 minutes, mashing them until they soften and are cooked.
Stir in the chopped tarragon and add the water, increase the heat to medium and cook gently for about 10 minutes, or until the sauce has reduced to a gravy-like consistency. Add more water if necessary to reach your preferred sauce consistency. Carve or shred your pork and serve with the crackling and plum sauce.