You need a special tool kit called a Hydrapack ram for undoing the top nut. As someone has said, you seem to be trying to remove the head and gearbox together. Is there not an old Wessex man around your parts, head removal is straight forward.
The Wessex schedule used to be a pain, because the head and gearbox O/H lifes were not the same so you would have to remove the head at 800 hours to change the gearbox, put it back on, and then remove it again at 1000 hours.