Place the ham hocks in a large saucepan with just enough water to cover.
Add the onion, celery, bay leaves, thyme, peppercorns, coriander, cumin seeds and vinegar. Bring to the boil, then reduce the heat and cook uncovered for about 3 hours until the meat is very tender and starts to fall off the bone.
Remove the two ham hocks from the stock and set aside until cool enough to handle.
Strain the stock into a clean pan and boil until reduced to about 600ml (1pint).
Soak the gelatine in cold water until soft. Squeeze out the excess water and stir into the hot stock. Allow to cool. Line a 900g (2lb) loaf tin with cling film.
Once the hocks are cool enough to handle remove the skin and shred the meat, discarding any fat and sinews. This is easiest to do with your hands. Place in a large bowl.
Add the chopped gherkins and parsley and season with black pepper. Toss to combine. Pack into the lined loaf tin.
Slowly pour the stock into the tin. You may not need all of it. Cover with cling film and chill overnight.
To serve, Turn out onto a serving plate and decorate with some chopped gherkins and parsley.
Most hams today tend not to be over salty and do not need soaking, but to be on the safe side you can soak the ham hocks for several hours before starting, if you have time.If you do not soak the ham it is a good idea to taste the stock before reducing. The salt will become more concentrated once reduced. If it is already salty enough for your taste, do not reduce further but measure off 600ml (1 pint) discarding the rest and add an additional sheet of gelatine to help set the stock.