Here goes, the recipe has a sauce with it but FIL brings his own which is similar. Beware though as its a bit 'windy' 
. It works with gravy too though, so you don't have to make the sauce. I don't usually bother with the celery and I don't always add thyme and sage but use black pepper, but it tastes very similar. I use more rum and sugar than the recipe states too, as it turns into a sort of paste when you unwrap it to cook, which helps with basting. I marinate mine on Christmas Eve and leave in the fridge till Boxing Day morning. This works on all sorts of pork joints, not just the ones specified:
Caribbean spiced pork
2 cloves garlic
3tbsp soy sauce
1 tbsp malt vinegar
1tbsp finely chopped celery
2 tbsp chopped spring onions
1.5 tsp dried thyme
1 tsp sage
2 tsp curry powder
cup of rum
2 tbsp demerera sugar
1.5 kg leg or boned loin of pork
salt & pepper
Sauce
2 tbsp butter or marg diced
1 tbsp tomato puree
half pint chicken or veg stock
1 tbsp chopped parsley
1 tbsp demerera sugar
hot pepper sauce to taste
Mix garlic, soy, vinegar, celery, sping onions, thyme, sage, curry powder, rum, sugar together and
add salt & pepper to taste.
Open out the pork and slash down the middle from the back, so that you have a large flat piece.
Add half of the marinade, including the bits of onion and celery and roll up (tie with string if you have it)
put the pork into a dish and pour the remainder of the marinade over the pork. cover tightly with cling
film and leave in the fridge overnight.
Spread a large piece of foil in a roasting tin, put in the pork and baste with the marinade that is in the
dish, roast in the oven for about 2 hours 190 c/375 f/gas 5. Then take out the foil, baste again and
cook for a further hour or so (until it is cooked how you like it)
To make the sauce pour the juices from the roasting tin into a saucepan and add the other ingredients,
simmer until reduced (you may have to skim it quite a bit to get the worst of the fat off!)