Place the gammon, onion, garlic and bay leaves in a large pot.
Pour in ginger beer until the gammon is completely covered and put a plate (or something heavy) on top to keep the meat submerged in the liquid.
Bring to a boil over medium heat.
Cook for 1.5 hours, then drain.
Remove gammon and allow to cool slightly so that you can handle it.
Discard any netting and pull off the thick skin (it should come off easily while still warm).
Score fat in a criss-cross pattern, taking care not to cut through to the meat or it will dry out.
Place gammon on a greased roasting tray.
Place tall the glaze ingredients except the grated ginger in a saucepan and bring to a slow boil to reduce the liquid by half. Once thick and syrupy, add the grated ginger.
Brush gammon with glaze and roast for 20–25 minutes, continuing to baste with the glaze while the gammon crisps.