It is years since I used a pressure cooker but the time depends on the size of the joint. A joint big enough for 4 people would take about 35 mins under pressure.
One thing that I learnt from a lady who used to teach cooking is to cover the gammon with water and bring to the boil. As soon as it boils take the meat out of the water and chuck the water away. This gets rid of most of the excess salts including saltpetre.
I hope this makes sense.
