I do a different gammon every year for Boxing Day, my husband's family only ever eat it here, and my FIL usually takes a chunk back home with him! I'm vegetarian, so have never even eaten any of it!
General rules:
Take off the plastic film AND the greaseproof papery collar around the outside.
I use a slow cooker, so ideally fill it so that it is at least two thirds of the way up the joint.
Cook for 4 hours on high/8 hours on low. Then the joint over, halfway through.
I usually do this the day before, and have left the joint in the liquid.
Then,
Drain.
Put into an roasting dish, and cover with something sticky.
Roast at GM5 for up to an hour. A couple of times, baste with the sticky juices. You want it to be caramelised and sticky.
The flavour combos I've used:
Coke/star anis to boil, dark brown sugar, honey and mustard to roast.
Orange juice and ginger wine to boil, orange marmalade and ginger to roast.
Apple juice and peppercorns to boil, apple sauce, honey and sage to roast
Orange and cranberry juice to boil, cranberry sauce, orange zest and honey to roast.
Good luck!