Cuban pork with a sticky citrus glaze

Name: Cuban pork with a sticky citrus glaze
Ingredients: 600g/1lb 5oz skinless, boneless pork shoulder; 50ml/2fl oz vegetable stock; 2 tbsp Seville orange marmalade; cooked rice or warmed tortilla wraps, to serve; 2 large oranges: grated zest of 1 orange, juice and squeezed shells of both; 1 grapefruit, juice only; 2 limes, juice squeezed, reserving the shells; 1 tbsp dried or freshly chopped oregano; 1 tbsp ground cumin; 6 garlic cloves, peeled; 1 tbsp soy sauce; 1 tbsp olive oil; 2 tbsp freshly ground black pepper; good pinch sea salt