I couldn't find a 5kg ham and only managed to get 2kg for Nigella's Ginger-Glazed Ham (Christmas, p102). How does that change cooking time? Do I need to boil it for full 4 hours, or should it be shorter?
The timings given in Nigella's Ginger-Glazed Ham recipe are for a large ham and a smaller ham will not need cooking for as long. Recommendations for cooking times for hams vary a little but if you allow the simmering time to be between 45 and 55 minutes per kg (20 to 25 minutes per pound) then the ham should be cooked through. If you have a ham without a bone then use the longer timings.
If you want to check that the ham is cooked through then either use an instant read thermometer or meat thermometer - it should read 71c/160F when inserted into the centre of the ham. Alternatively remove the ham from the cooking liquid, pierce it with a knife and check that the juices are running clear.
Once it has had its simmering, the ham then needs 20 minutes in a hot oven to cook the glaze and this part does not change for the size of ham.