The cooked meat needs to be thoroughly reheated until piping hot and should never be re-heated more than once. Shred 600g (1lb 3oz) of ham and wrap the leftover meat in foil or place in a lidded airtight plastic box and store in the fridge. Use within 2 days. Preheat the grill to high. Halve the burger buns and toast the cut sides until browned. Heat the oil in a large lidded pan over a medium heat and cook the onion and celery for about 8 minutes to soften. Stir in the garlic and paprika, cook for another couple of minutes, then stir in the barbecue sauce, tomato purée and sugar. Add the shredded ham and 3?4tbsp water and stir everything together well. Turn down the heat, cover and cook for anther 5?10 minutes, stirring occasionally until heated through. Taste and season if needed. Divide the ham mixture between the bottom halves of the 4 toasted buns, top each with cheese and a handful of the shredded lettuce. Place the top of each bun in place and serve hot with coleslaw and potato wedges, if liked.