Why soak it... in the old days it was to remove the salt from the brine.. Nowdays they just barely cure and sell to eat so no need to soak. Just remove from bag, boil - no just about to boil (just a few bubbles at a time) and then grill a bit when finished with some sugar, port and honey to glaze..
Just found this...
FROM THE NIGELLA TEAM:
If you live in the UK then you probably do not really need to soak your gammon in advance as the cure on most gammon hams is relatively mild. However it is best to read the instructions that come with the gammon and follow the advice from the supplier. If you are still unsure then cut a small piece from the gammon and simmer it for a few minutes in a small pan of water, until cooked. Cool slightly and taste the cooked ham, you can then tell if your ham needs presoaking or not.
If you do want to soak the ham then allow 8 hours for a very small joint and up to 24 hours for a large one. Make sure the gammon is kept cold during the soaking and the water needs to be changed every 6-8 hours (change halfway through for smaller joints).
If you live in the US then uncooked hams tend to be very salty (quite often they are country style hams which are cured with salt for up to 3 months) and we would soak the ham for 48 hours before cooking, changing the water every 8 hours. Uncooked hams are not common in the US but you can buy a fully cooked ham, reheat it following the supplier's instructions and then glaze it for the last 20 minutes of cooking time.